You've got such a difficult decision to make, OP! I don't envy you and I've never been in your position. I don't have children either. But I will make this post anyway in the hopes it will help you in some way.
I'm glad you have your mother to confide in and support you IRL.
I hope your partner does not display the same awful behaviour when he comes round tomorrow. Let's hope it was only due to shock. But two strikes and he's out.
IF you decided on abortion (for your own reasons, not as a result of being bullied by this man) would you want to carry on with him? Knowing he likely wouldn't have supported you in having the baby?
IF you did carry on the relationship, do you think he could cope with being a stepfather to your children? He sounds very anti-children indeed. He doesn't even want his own child, at least not right now. Could he cope with your children, do you think?
What I am trying to establish is, do you think he sees you all as one package? Or does he just want to pursue a good time with you and he isn't really taking the reality of your three children into account?
I agree with the pp who said that he will likely promise anything now just to secure an abortion. He may drop you after he has achieved this aim. I really don't know. Only you know him - or at least you thought you did until the other day!
I would be very very careful in factoring him into your decision.
Assume you'll be doing this on your own. Then decide. Get any counselling you can in order to help you decide. Write out the pros and cons and try and get your head straight about this.
There is no doubt that having an abortion would be easier in practical terms - plus if you split with the guy you never have to see him again, etc. Money will be easier. No more newborn stuff.
If you are very against abortion, I think that could override practical decisions. Practical stuff can always be worked out in the end - no babies would be born if we waited for the perfect circumstances.
Time is still on your side to think this through.