You let your opinion known, he knows that you are against, so by yielding to his request you will show that his needs - as foolish as they may seem to you - are actually important to you.

Also, the fact that Apple and Google stores have different content is a very big issue for kids. Someone picks up a new game, comes to school, shows it off, everyone gets it, plays it for a day or a week, and then moves on to the next one. If your kid is on another store than doesn't have this game, he is excluded. Repeat this few times and this will leave a mark on him even without someone making fun of him because of that. And the Apple store does get better games first.