My younger brother is turning 14 tomorrow!
We've always been a bit concerned about what he plays, but had thought about getting him Fable III as I've played it and it's not too bad.
As Rich said, BFBC2 is a very good game. If it's on Xbox let him know that he's more than welcome to add me on XBL (username = GT) and I'll see if I can find the time to play.
Also, if he is enjoying Halo 3 I'd certainly get him Halo Reach and/or Halo 3: ODST
If you're a fan of even one of the Halo games you should at least play through the campaign of the rest of them
(One last thing, I won't drag on all day!) If you do decide to get him BFBC2 (Battlefield Bad Company 2) and he enjoys it, a new(ish) game that was released last year was the new Medal of Honor game. It's practically a Battlefield Bad Company game. - It was made by (most of, I think) the same people and certainly the same company who made the Battlefield franchise.
MOH (Medal of Honor) is a good game, although for me the playability of it wasn't that great.
I've got a bit of time over the summer now, so hopefully I'll be getting this all down on my forum haha
Sorry if I went on a bit! O_o