Advanced Wars is a great TBS strategy series, if he has a GBA, get him Advanced Wars 2: Black Hole Rising. That game was Nintendo Hard on so many levels and took some real smarts to beat.
If you have a DS, its a toss up between Dual Strike and Days Of Ruin. If you like tons of different units, over the top tag team Commander abilities, and an insane storyline, get Dual Strike. If you like a smaller, more balanced unit roster, commander powers that dont break the game, and a "darker and edgier" story, than you will want Days of Ruin. Difficulty wise, I would say Days of Ruin is the harder of the 2 DS games, but thats just a personal opinion, they are both great games.
Theres also a Free to Play TBS game online very similar to Advanced wars called Battledex
http://battledexgame.com/welcome.php
It has tournaments, leauges, co op maps, and a really cool community. You could get him that and buy him a few booster packs and I can almost guarantee you that he would love that game as well.