I agree with a pp. If your DH was that worried about another pregnancy he should have had the snip
This sort of response really annoys me. We have no idea how OP got pregnant. It might very well be no accident. They might have discussed it and he might have said he only wanted two before. Considering how happy OP seems to be and clearly not wanting a termination, it wouldn't be surprising if even if it really was an accident, he didn't believe it.
In the end, it all comes down to what they've discussed before, about how many children they wanted, what happened if there was an accident. They might have agreed to a termination then, in which case, why would he have insisted on him getting the snip?
It happened to my colleague, same situation. He was always suspicious that the pregnancy wasn't really an accident, but he found out that his suspicions had been correct when after a boozy night, his wife's best friend let it slip that she'd deliberately stopped the pill and had planned to lie about it because she knew he would never had agreed.
He never got over the betrayal and left her. Worse, he struggles to bond with the third child. He knows it's not his fault, but he can't let go and is in counselling for it.
I really hope OP that your OH will come around, but it is not fair to dismiss his feelings. Him not wanting you to go ahead with this pregnancy can be as strong as your wish to do so. Yours don't tromp over his. You really need to talk and support each other.