We (dh & I) have had this problem. We had it for several weeks, several months ago. It wasn't stage fright tho, as dh & I have married 16yrs.
It started as one failed attempt & then continued from there really. He was fine getting an erection, maintaining it during foreplay etc but as soon as it came to penetration, it was gone. We tried all sorts, but nothing seemed to help. I am pretty sure the pressure of the first failed attempt knocked him sideways, tbh.
He could come through masturbation & I had no problems making him come. Just couldn't get it to happen 'together'.
So we just stopped trying. Did all but the penetration. After several weeks it seemed to have fixed itself. Your posts do all seem to be about penetration, even though you have said you can wait/do other stuff etc...asking about cockrings, not using condoms as a solution etc. Yes, I know you want a solution, but there is an awful lot of pressure from you on him in these posts & I wonder if he gets that when he is with you? Regardless of your demeanour at the time. My dh always picked up on my disappointment & I know that made it more difficult for him. Which is why we took penetration off the table for weeks.
I don't know if your man's problems are down to him being gay-not-bi/or stage fright/a medical problem/a more long term problem. But would it not pay to have a conversation about it, a bit more than 'oh I am bi & it is stage fright'. Because regardless of anything, this is your future too & surely it is better to know what you are dealing with now, than flogging a dead horse for the rest of your life, only to find out he is gay 20 odd years down the line?