Additionally your instinct about the 5.56 being a good place to start is a good one.
Build a 5.56, play tinker toys with it, figure them out a bit, then go from there....
Given what he's said through the thread, I agree, largely because of ammo availability and price. Anybody getting started in shooting long range should plan to shoot a lot, and that's hard to do if your ammo costs too much. 5.56 isn't the best long range round by a large margin, but a decent rifle with a bucket of 75gr or 77gr ammo and plenty of range time will do a lot more good than chasing the best and shiniest long range cartridge.
Of course it's pretty easy to swap a barrel later or just build another rifle or upper when he's ready for the next step.
Kellory - for clarification, both the 308 and 6.5 Creedmoor require a large frame AR, which is a different beast entirely from the AR15. It's more than just a different lower; it's a whole different gun, although some small parts interchange and they work the same way.
