We've been dating for a few days but I don't want him to break up with me

A nerf gun.

Well I'm close to you age and my ex boyfriend LOVED a homemade card, teddy bear, and a kiss...
But it depends on how much you want to spend too.. but the best gift to get any boy is Call Of Duty. (:

You are "dating" a 12 year old? Are your parents aware of this? Twelve is really too young to be in a one on one dating relationship. The likelihood of his breaking up with you at some time in the future is pretty high. And if he breaks up with you because he doesn't like whatever birthday gift you give him, then he's not worth keeping in the first place.

You're young, and it's not likely that you two will be together forever. One thing I can tell you. You can't keep someone in your life by buying them gifts. If you have to do that to keep the relationship going, then you don't have a relationship to begin with.

Why not just be friends with him? Go out in groups, have fun, but don't start dating. You're very young, and so is he. Let yourself be what you are, a child. There's plenty of time to grow up and start dating.