OP, this might not be the most popular opinion you'll get on this thread, but I believe that euthanizing him wouldn't be "the worst".
He is lucky that he had you in his corner, doing the right thing for him. Not every dog in his situation has that. Sometimes doing the right thing means accepting the reality of the situation and the reality is that he is a very difficult dog to find a responsible home for. Sometimes, despite the very best efforts, a dog just can't find the right home.
The worst end for him, in my mind, would be for him to get shuffled around from one home to another, with each one getting worse and worse in terms of his quality of life.
You could invest in some training with a professional, and work with him on his issues, if you haven't already. The down side is that you could be looking at a long and perhaps expensive effort that still finds him with you after the birth of your baby. The upside is that you may be able to get him to the point where he doesn't nip and bite the people who come to look at him.
Sometimes the most humane, responsible thing we can do is give them a calm, compassionate death. It is never an easy choice, but sometimes it is the right choice.
Good luck with whatever you decide to do.