I am very non maternal. Never, ever going to have a baby. I hit 29 and hormones raged. The never going to have babies are now aged 24 and 18! So, in a way I get why people say 'things will change'. I am not maternal at all in the wafting around going gooey over babies kind of way so I know what you mean by never wanting kids.
The difficulty you have is that you have a known thing (your partner) whom you fear you will lose and an unknown thing ( baby) that you fear you may regret losing. Regret, fear, worry and anxiety are all making you want to hide. I would gently point out that you have let yourself get to 17 weeks possibly because it is nearing the point of no return because you didn't really want to make the abortion decision much much earlier. Now that point of no return is here you are panicking, and frankly, you are quite normal in feeling this panic. It's a bit like you have signed up for a sky dive for charity and you are at the point where you've got to jump out of the airplane. The person you thought was going to jump with you and you'd hold hands has backed out and is now sitting in the seat at the front with his headphones on and you can see the tiny dots of family and friends down below expectantly looking up at the plane. Everyone would be shitting themselves. You have to weigh up the fall out of jumping or not.
I think an abortion at 20 weeks would be very distressing. Even Without the whole emotional thing, it would be a very medical procedure. Personally, I don't think I could continue a relationship with someone who was essentially sitting at the front of the plane with their headphones on occasionally yelling 'not my problem' at me whilst I had to cope with the physical and mental realities of a 19 or 20 week abortion. I presume you have had scans, etc, too. He will expect ( because he just won't 'get' the whole hormonal, visceral, medical fallout of an abortion) things to return to normal with you guys. I cannot see how it can.
It is very difficult when everyone is excited. Do his parents know he has said he is not being involved? What are their thoughts on this. I get the sense from your post that if he said, I'll take my headphones off and jump too, then you'd go ahead even though you are scared witless and are sort of doing this for other people. Having reread your post, it is lack of involvement that is impacting you. It's fine to not feel overwhelmingly attached to your baby and subconsciously you probs haven't let yourself. It's also fine not to like kids that much - mine are fine but other people's....nah. Perhaps you are expecting to be some earth mother overwhelmed with love and attachment fuelled by family excitement. They can be excited but you are going to deal with the reality of the situation and it is ok to think 'oh shit' on a frequent basis.
Just best of luck in whatever you decide to do. Make sure it is the right thing for YOU and be aware that this is a life experience just like big things like bereavement, marriage, moving country, divorce, changing careers, education...and just like those 'big' things you decision now will change the course of your life majorly or slightly - who knows - but there will be change physically and emotionally.