I agree with everybody here. Any of the known name brands will do fine. Rock River, DPMS, Armalite are all excellent choices. I have shot all of them. I personally own a DPMS. I have replaced the upper with a Double star free floated Match Barrell. I liked the upper so much, I brought it wiht me to Iraq.
Sir, your son really won't be wrong to buy any of them. For the most part, with maybe some minor customised parts or company specific parts, they are all about the same. Until you move to some of the custom AR-15's like Les Baer, that is just how it is going to be. So buy one from any of the companies. Nice thing about them is that you can always upgrade different parts. You can customize it. It is super versitile.
Somebody asked about the AR-10. It is a super Rifle. Check on the price 0f 1500, but a great rifle. I went to Sniper School inthe Army with a guy who competes with one and has done very well. I have no doubt about kiling pigs at 400 yds with it, and I would be you could do it almost twice that far with one.