Well, first, if you're dating, then you aren't a step mother. You're the woman their dad is dating. So there's a problem that's been overlooked right off the bat.
Second, it's hard to know if he's really bragging or if you are overly sensitive about the issue and interpret statements as bragging that are not intended as you took them. Care to provide some specific statements he made to help clarify?
There is some truth to his last statement. Most people without children, or without a LOT of experience with them, don't really know what they're doing. But then again most parents don't know either.
Now if he's saying this in a critical way then that's a problem. But again, it could be that you're hearing it in a way it's not intended so getting specifics will help determine if he's being a jerk or you're being overly sensitive.