I wouldn’t worry. I was probably six feet tall at 15. I topped out at 6’2". I stopped growing at 17.
Really, it seems to me that he’s tall, sure, but not freakishly tall, not tall to the extent that some kind of health problem is indicated.
I’m 6’ 7 1/2". My only problems are not being to buy pants or long-sleeved shirts off the shelf, and a few bruises on my forehead while I was growing up. (Because I was 6’6" by the time I turned 17.)
Not acromegalic, no connective tissue disorders, no Marfan’s syndrome. Just tall, probably for reasons of genetic heritage. My health is fundamentally fine.
As much as the opinions of random strangers on the Internet should matter to you, I think you’re worrying too much. Because (A) your worry borders on Internet hypochondria by proxy, and (B) you haven’t indicated whether the lad’s parents share your concern.
Also, the whole reason seats are adjustable in cars is because people come in different sizes. Yes, taller people might not be able to plop right into a car seat however it’s configured. That’s fine, because that’s why the seat moves.
As a 5-foot-8 man, I don’t think this is an issue; he should keep growing as long as he can. 
And, judging by my 6’5’’ uncle’s experience, hearing ‘‘You’re tall!’’ every day of his life.
My son turned 15 one month ago. He is 6 foot tall. It never occurred to me to worry about his height. He is tall and has a large frame. He may grow a few more inches and he may not. We will see.
No, shame on you for this nonsense. My son was 6’ 3" at 15 and is now 6’ 5" at 25. I was probably around 6’ feet all to 6’ 1" at 15 myself and I’m about 6’ 3". Are you really so tiny that people over 6 feet tall at 15 are giant freaks of nature?
Good God! What kind of rollerskate are you driving that a not that tall 6’ 1" person can’t sit upright?
Yeah, 6’1" at that age is near the top of normal height. Note–top of normal height. If you don’t see anything to suggest some growth problem (certain exaggerated facial features, grossly disproportionate limbs or extremities etc) he’s probably perfectly healthy but tall.
I was 6’0" when I joined the wrestling team in 8th grade. I didn’t grow much at all for like 2 years, then around the summer between 10th and 11th I noticed I had started growing again. I was 6’5" when I was measured in a military physical at 18 and never grew a bit after that.
I had a friend who was 5’7" at 18 when I met him, by age 21 he was 6’0"–and that’s pretty unusual, while you can still grow until your early 20s growth of a few inches is pretty rare, most of the hormones are lower by that age.
Two famous wrestlers who had/have acromegaly, Andre the Giant and Paul Wight, are good examples of people with obvious gigantism of a medically concerning nature. Andre was 6’3" and 240 lbs at age 12. His top height is believed to have been anywhere from 6’10"-7’, but more concerning than his height is his overall sizes. Andres wrists and hands, his legs, his chest and shoulders, were all significantly broader/thicker than any man’s should ever be. While Shaq likely is a few inches taller than Andre ever was, Shaq would look small next to Andrew because Shaq is a proportionate, healthy, very tall man. Andre was not, and he died young because of the strains put on his body by a never ending secretion of excess growth hormone.
With gigantism caused by acromegaly, you hit your “genetic potential” in terms of height, and then you start to “overgrow.” Your body is no longer growing longer, but bones get thicker and even organs show signs of overgrowth, and it does terrible damage to the body.
Paul Wight had all the same symptoms and is presently about 7’, notably and this is because of the huge amounts of growth hormone, he said he could eat 6-7,000 calories a day as a teenager and he didn’t have any fat on him (not magic–the high growth hormone was fueling growth beyond what is normal and used that food for fuel). He went to Wichita St. at 7’ and 300 some pounds and very little fat. He also had, facial features and limb features that made it obvious he wasn’t a healthy seven footer. Luckily for him, by the time he came of age science had advanced, a team doctor noticed his features and told him he suspect he had gigantism. They found a tumor on his pituitary gland that was otherwise benign, but had likely formed when he was a little boy and had been pushing on it ever since, causing excessive growth hormone secretion. Paul has avoided most of the health problems Andre had because he was fortunate enough to have the pituitary tumor removed right in early adulthood, before it would’ve started causing more serious problems.
Also, being tall isn’t that bad. Coach airline seats kinda suck and certain model cars are unpleasant. Having gigantism is usually much worse because you aren’t just long, you’re big in other, less normal (and less accommodated ways.) For example, Andre the giant had to travel a lot in buses and airplanes, and says he often had to be allowed to sit in the aisle of buses or in the galley area of planes on the floor (he’s probably lucky he lived before the age of TSA and stricter flight rules), unless the plane had large first class seats. He said in one interview he wasn’t able to attend movies because he couldn’t fit in any movie theater seat, so his only option was to stand at the back wall for the duration or sit in the aisle, which usually caused unwanted notice (unlike many celebrities his size made it so he was never able to blend into a crowd or get any rest from curious onlookers.)
You can find pants and shirts for real tall people at big and tall stores, but guys with true gigantism and their insanely thick wrists/arms/legs often cannot be well accommodated even there, and will have to have every piece of clothing they own custom tailored if they want it to look anywhere close to decent. Link – to show what I’m talking about, this is a picture of Andre’s hands next to Chuck Wepner’s. Wepner was a journeyman boxer in the 60s/70s who himself was 6’5".
I’m more curious where the OP is from. When I was in high school my offensive line averaged 6’2" my senior year and we had a sophomore and two juniors as starters. Even as an adult I have worked on engineering staffs where the shortest person was 5’8" and the average was over 6’. Hell I’m 6’6" and I wasn’t even the tallest person in that office.
I was 6’ in 6th grade and was 6’2" when I entered high-school and reached my max height my senior year. My growth was pretty steady at a inch per year once I got to 6’ and I don’t remember and big growth spurts that my friends talk about getting.
Generally speaking being tall is awesome I’m all torso (34" inseam) so finding shirts can be a pain but most shoes.come in up to size 16 as fairly standard. Heck I’m hoping my daughter breaks 6’.
Yeah, never having seen a kid that age be 6’1" would be strange unless we’re talking Vietnam or something. In my high school we had two kids I was pretty good friends with on the basketball team, one was 6’8" and the other 6’9" (they were brothers actually, the 6’9" kid being a year younger.) We had another guy who was probably 6’7" and 385 lb in HS who was on the offensive line (despite his huge size he wasn’t that good, but he could push around people at low level HS football.)
Not that we were a high school of giants, average height was definitely around 5’10-6’ for men, about typical, as I recollect. But we had enough tall guys that one wouldn’t be a “freak of nature” for being tall in that school.
OP should say where he is from as some countries like Netherlands have so many tall people it norm.
The countries like Vietnam or the Philippines have really short people.
The OP’s profiles says she’s from Australia, average male height of 5’ 9". Her nephew is going to be just fine.