It's one of the toughest decisions you will ever have to make, both individually and as a couple. I'd strongly suggest seeing if you can talk to a counsellor both separately and together, go through your budget with a fine tooth comb and really assess if you can afford another child and discuss your findings with him.

Ultimately it is your body and he cannot force you, but if you continue with the pregnancy and he's not on board, it may breed significant resentment and come to breaking point - could you face that? Likewise can he, if you do terminate?

There is no easy answer to this one, and really, no one else can tell you what is going to be right for you and your situation/family.