If you do not have kids...
Tell him directly, with a friend there if you feel he may blow up at you instead of really listen, how you're feeling. If you want it to work it out and stay in the relationship, tell him. And then say, without making it sound as though it's an ultimatum, if things don't change, then you have to leave. He's probably not all that happy if you're not happy, so it's for the best.
If you DO have kids...
Tell him how you feel and why, but make sure he really listens. Don't get emotional. If it turns into a screaming match, then just stop and put it into a letter.
Insist on going to marriage counseling. If he refuses, then you need to think long and hard about the environment you want your kids raised in. It's best that they have two involved parents. Do whatever you can to work things out between the two of you.
If it gets worse, then contact a lawyer, have a letter drafted up, include the amount of child support he'll have to pay, and see if he changes. No change, then the two of you have made a mistake and your children's life will likely become yet another part of divorce hell.
Hopefully your children (if any) won't be forced to deal with you and/or your husband's immaturity. Sorry to be blunt, but ultimately that's what it comes down to, doesn't it?