Just because his mom allows it, does not mean that it is appropriate or that you should compromise your beliefs to buy him a gift. I think that video games especially the gaming kind cause a lot of harm, and you are wise to be very picky about the ones that they play. I did not start out so careful, but am now, and it is much harder to get stricter than it is to start and stay there.
The problem is that kids can not unsee what they see, and the video games are so realistic anymore that it desensitizes them to reality. The second thing is that if you watch the games while they play it will probably make your head hurt, if kids play games for hours a day, how on earth can they ever hope to concentrate at school or in a normal paced world. And the third thing is, my husband raised a step son who is now 26, he was totally into video games, and it has now transferred to the online role playing games, and it would take me three pages to tell you just how badly this has affected his life. And he is just now starting and I stress starting to get how truly screwed up he is.
Hope this answer helps you to stand firm, get him some roller blades or something that he can do to get away from a screen :)