Ok, I'll shoot.....

We southerners have a tendency of running the gamut in sizes, particularly in the larger sizes. With that being said, I have known Clydesdales and Athenas that ride carbon fiber bikes. The trick to this is getting properly fitted bikes and then splurging on nice, bomb proof rims. A rim with more spokes will work better for the larger rider or a bomb proof rim with fewer spokes. I like the Mavic Ksyrium rim for road bikes and Mavic Crossmax for the mtb because not only are they super sturdy, they are virtually maintenance free. The spokes won't need truing for at least a couple of years. One gentleman who is 6'4" and 280 lbs rides with the Ksyriums and swears by them.

Hope this helps and it's nice to know your son is getting bitten by the cycling bug.