Well, it depends on what kind of boy he is and what his interests are,but I would buy him a video game cause it works on my cousin.If u can't find anything tell him he gets nothing cause when u grown up (like he thinks he is) u don't get presents anymore trust me i've learned it the best/hard way.
My kids love video games, but I feel they spend too much time inside playing them. My son just turned 12 a month ago, I know it's a hard age and gender to buy for. He got a super soaker squirt gun that he has had a lot of fun with. To be fair, pick up some small cheap squirt guns for the other kids who will be around so they aren't defenseless, but the birthday boy will have the advantage.