You can buy skins mostly, they , make the character you play look different, anything else you can also buy by earning points in the game (although it can take a while). I'm sure he'll be happy with it ;). The fact that you thought about LoL in the first place will probably be enough to make him adore you haha :).
Just make him a home made card expressing how much you appreciate having him in your life. Better than buying something because you think it's an obligation. If you're going to do valentines day then do it right. As in make it a day about each other not about material gifts.