As a parent it is your job to set the rules of the house. If you don't want him playing too many games, define how much is allowed and punish him if he disobeys. You are not his best friend, you are his father. It doesn't matter if he buys the games whit his own money or what he wants to do when he grows up. You set the rules he has to obey. That's the parent/child relationship. Everything in your house is under your control, even if your kid used his own money.
For example. Here are a few rules you could implement.
1. You will confiscate his gaming stuff if his grades drop below 3.0, until they are back above 3.0.
2. He must finish his homework and chores before he can play games online or offline.
3. He gets 1 hour of gaming on school nights, 4 hours on weekends.
4. Games you don't approve of can be confiscated at any time until he is 18 or you feel he is mature enough to play.
As for his taste in games differing from yours. You need to let that go. FPS is the most popular genre today for a reason and it's not a fad. You need to either deal with that or stop worrying about it.
