Best to be honest and up front no matter how hard it is (and is probably)going to be. I think he'd rather you be honest with him than hide it from it from him.
Like you said...it takes 2 and he needs to take responsability for his part in this. In the end, the choice is up to you... after all, your the person who is growing a new life inside of you.
Hope it goes well. We are here if you need to chat.

CONGRATULATIONS though