This is extremely unfortunate, however we are all humans, and some times feelings change - this is something you can't control, while very ill timed, atleast he was honest with you, instead of just seeing someone behind your back (Not knowing if he did so before telling you) - would you rather he stay with you without loving you? It sucks, but it is better than him staying with you just for the kid. try ending it on good terms, he'll still be the father even if you're not together and just because his feelings for you changed, he could still be a good father, I know of many people who split up on terms like yours, who stayed civil towards each other continuing to raise their kids together. Might not be very kind on his side, but I'm sure he did what he felt he had to do.