Yep, too much TP.
You’re right that we don’t really know how much TP he uses, although you’d think he’d know just by having kids and a family, and living 50+ years. It’s also pretty embarrassing for him, so if he even suspected too much TP, he’d probably use it more sparingly when out of his home.
You dont have a low flow toilet, do you?
No, and he doesn’t always have to “go” when he comes over (which might be like twice a year). I’ve just known him for decades. So like while the rest of my friends may have clogged like 3 toilets in our lives combined, he’s probably at around 20 all by himself. Not exact numbers, but just to give you a sense.
I use to much toilet paper, and i rarely clog toilets. When i do, it’s because of an unusually large turd, not the paper.
(I did have one friend with a terrible toilet, and i often clogged that one. Fortunately, he left a plunger next to it. But in general, i don’t have an issue with clogging toilets.)
I would think the variety of bacteria living in your gut has a huge impact on the quality of your stool. Those little things can ferment fiber and regulate water.
~Max
I eat a lot of meat, and don’t clog toilets. So while eating a lot of meat when you’re not used to it might have that effect, it seems unlikely to be the sole cause if it’s part of the ordinary diet.
I once shared a household with a couple of large young men eating high fiber diets. The size of the turds was definitely an issue.
Yeah, I would think that high fiber diets would be more likely to cause clogs. Fiber absorbs water and, depending on the type of fiber, expands like a sponge or turns to a gel. Although I must say that I’ve been drinking a heaping spoonful of Metamucil every night for the last 30 years, and I’ve literally only once clogged a toilet, which I think was more an issue with a previous user of the toilet.
This is probably not the primary issue, which is more likely to be heavy TP use (despite denials that this is the case), and/or large, firm poops, but…
If one or more of the toilets in question were manufactured in the mid '90s to the mid 2000s, it might be in part a toilet issue. That was when the U.S. first mandated “low-flush” toilets for homes (which use 1.6 gallons of water versus 3-3.5 or more in the old ones). The first-generation low-flush toilets were often the same design as the old ones, but just filled to a lower level; they tended to have weak flushing, and more easily clogged. Toilet manufacturers eventually figured out design improvements to improve the power of a flush despite the lower water volume.
Interesting, as I think most people old enough know the difference. There is a clear lower force of flush with those types of toilets. Thankfully they don’t seem to be around much these days. Although I still see some that have that half-flush option, which makes no sense to me.
For liquids only. Makes perfect sense.
My parents still live in the house we moved into in 1975; at some point in the late '90s, they had to replace the toilet. It’s still one of those weak low-flush models, and it will clog if you just say the word “toilet paper” in its vicinity. 
I mostly see those in public restrooms, and as @commasense notes, the “half flush” is meant for flushing the toilet when it contains no solids.
I think it just dilutes the toilet water, but doesn’t completely “flush” it. So a higher proportion of urine still remains vs. a full flush.
I have the internet to thank for teaching me about the “poop knife.” I had never heard anything like it previously. Apparently you have a “poop stick.” 
lol, my first thought as well.
Gives the ubiquitous term “hot shit on a stick” a new meaning. Don’t it? 
He probably means he doesn’t use more than he needs to get clean. That doesn’t mean he doesn’t have unusual cleaning needs.
When my son was being potty-trained, I taught him to flush every two wipes, when I noticed he was taking a bit extra to get clean.