I can totally empathise. I'm pregnant with no 3 now and it was entirely unplanned and if I'm honest I was distraught about it until about 20 weeks, I'm 28 weeks now. I didn't want another, and we have no space 😅
You're still very early, feelings are going to be all over the place, it took me a while to come around and my OH was fine with it from the beginning.
I'd probably expect he might take quite a while to come around to the idea of two children, maybe even until baby is actually here. But you said he's a good dad and I bet he'll love baby to bits when he/she is here, and neither of you will ever be able to imagine life without that baby again.
The transition for me between 1 and 2 was quite hard, but it was made worse by postnatal depression and that I didn't seek any help or support for it, or even talk to any family about it. Definitely make sure you've got good support in place in case you end up with PPD, and for your husband too as men also get PPD.
There's 18 months between my first two and my eldest couldn't talk yet by the time her sister was born which made it difficult.
There's a book called "there's a house in mummy's tummy" that's really nice and I've been reading it to my 3.5yo and 2yo this time around. My youngest will be 2y 4m when this baby is here, and that's a smaller gap than I'd wanted if we ever had a third.
I'll also say neither of you should feel bad or guilty for not feeling good about this pregnancy, it doesn't mean you'll love baby any less when they're here.
With my second pregnancy, half the time I was in a state of "what have I done? Why am I doing this?" And my second daughter is a delight and now I couldn't imagine not having her here.
I think it would be healthy to just talk about the situation a lot, let him know you think he's a wonderful father and that even though this situation isn't ideal that he's going to do a great job.
It could take him a while to come around though, I was surprised how long it took me to come around to it and I'm the one who can actually feel the baby.
I won't lie, it's much harder having two. It makes having one look very easy 😅 but I think it gets easier after that first couple of months.