Personally, I don't think human nature will EVER fully allow you to trust him implicitly again. I think that you'll always consider the possibility and that will be seriously hard to handle.
Question: Did you find this out or did he tell you?
If he told you there's some room for hope (he has some exisiting genuine consious there)
If you found out, would he have ever told you? (After an entire year; likely not)
And it's different if it was once or twice, but a year is a committment to something.
I would suggest you evalute the situation for your son (as hard as it will be to sacrifice your feelings) but whichever you decide will have a tremendous effect on your son's entire being and life.
I wish you the very best of luck in whichever you choose. And congrats and being a mommy-to-be!