There are two trails down from the South Rim, which is the main part of the park. The South Kaibab trail is shorter but steeper, narrower, and more rugged. It’s a little bit over six miles long. The Bright Angel Trail is more gentle and wider. It’s 9.7 miles long. Because of the difference in steepness between the two trails, it’s pretty common to hike down the South Kaibab trail and up the Bright Angel Trail. On the other hand, the Bright Angel Trail is also the one that the mule trains go up and down, so you have to watch your step sometimes, if you know what I mean.
Strenuousity and time? That’s hard to judge for somebody else, but I hiked up and down both trails (not on the same day) when I was seven. (My sister was five!) Presumably he’s in good shape and has done some hiking before? My parents have both hiked in and back out on the same day, but that’s a long day–if your dad can make a reservation at Phantom Ranch at the bottom of the canyon, it’ll be much more enjoyable trip. Oh, you can also get a permit to camp at the halfway point on the Bright Angel Trail. In general, I personally find hiking downhill to be harder than hiking uphill.
December’s a great time to go because of the weather–not unusual to have snow on the rim, but much more temperate by the river! Summer can be just brutal.
Equipment: good hiking boots, comfy layers of clothing, food and water for the trail. I can’t remember if we brought sleeping bags for the bunkbeds at Phantom Ranch, or if they give you sheets and blankets. I’m pretty sure they fed us, though. Small first aid kit. I don’t remember what, besides a warm layer, you need to bring in order to be safe, but the National Park Service’s visitor center ought to be able to tell you over the phone what they recommend hikers bring. The trails are pretty well travelled, of course.
Hope he enjoys his trip! It’s really fantastic on micro and macro levels–the canyon itself is amazing, and I remember being able to see lots of fossils in the upper layers of rock.
This site looks pretty helpful: