Well, you either keep quiet about it and be happy now and miserable later, or you tell them about it and be miserable now and miserable later.

That's the way it is. You may never be accepted or be welcomed into the family (then again, they may grow up, and I hope they do).
If your boyfriend can deal with what he is/will be treated like, then he's for sure a keeper. That kind of stress is tough on a relationship and the whole mom/son/daughter-in-law (or future daughter-in-law) dynamic is a tricky one...