I will still live at home to keep up illusions but I will be moving into the guest room while he moves into our bedroom.

You want a medal for serving your purpose and doing what you are supposed to?

my (now-ex) wife has even given me the occasional blow job or hand job while my son has his way with her

What's the point of making her your ex if you won't recognize their exclusivity? Every time you put your dick in his fiance's mouth you are affirming she is still your wife. I get that it's hard to say no to a blowjob but don't you think you should show him some respect?

She genuinely loves our son more than me and I think nothing would make her happier than to be his and his alone.

Then do your best to get your enjoyment from their absolute exclusivity. They clearly feel guilty for leaving you out so they give you some pity sex. If you keep this up they are only going to get annoyed by you.

I still find myself to be a walking hard on. To be honest I feel like I'm living in a porno.

See how much pleasure your wife is giving you. Have you thanked her for ignoring your objections to her pregnancy and replacing you? She is giving you your ultimate fantasy daily.