According to Nosler Load Data, the 7mm-08 will push a 140gr Ballistic Tip, Accubond or Partition to about 2950fps using Big Game. In factory loads, about 100fps slower.
The 6.5 Creedmoor will push the same weight bullets to about 2730fps handloaded with IMR 4350 - about 2710 in factory loads.
The Creed will launch a 120gr bullet to about 3,060fps, the 7mm-08 to 3,260 when handloaded. Similarly, about 100fps slower in factory guise.
If the 200fps speed advantage is important to you, choose the 7mm-08. If you think you'd like to load 175gr bullets for some reason, choose the 7mm-08. Otherwise, IMO, there is no practical difference. High-quality factory ammo with premium bullets are available just about anywhere for each.
I haven't shot a Creed, but I have shot a 6.5x55 and a 7mm-08, and to me, recoil is a wash - fit of the individual stock seems to makes more difference in perceived recoil than the choice of cartridge.
If he REALLY thinks he is going to be shooting to 600-700 yards or beyond, the 6.5 will give a slight advantage with higher BC bullets. But remember, to take advantage of the difference, he will have to train, not just do some casual shooting for fun. - and in that case, he'll probably end up wanting a dedicated long-range rifle and an appropriate scope.
If it were me, I'd buy a Tikka T3x stainless in 7mm-08, and keep the 700 as a backup or loaner, with a slip-on recoil pad for adult use.
