I also think it would be a good idea for your son to move out. Find a couple roommates to share costs, etc. It would help him mature more and self-confidence comes from that.

And, I think it would be good if your son spoke directly to his father. You acting as intermediary means that each of them are talking to you, not each other, and your relationship with each affects those conversations.

And, speaking from experience, no one deserves to be given an ultimatum. When most people receive one, it instantly gets their backs up because it looks threatening, controlling and manipulative. It will make the situation worse. Instead, decide what you would like to do, invite your husband to have a conversation about that, and when/if he is unhelpful or shuts you out, go do what you've decided to do.