It seems to me the reason he’s so “ inexplicably unhappy” has to do with the lifestyle he’s living. At the risk of sounding like a complete prude, poly relationships require a complete lack of self regard to such an extent that I find it hard to see them as healthy.
As soon as he mentioned his announcement gf as already having a bf, which he’d had to make an impression on in some sort of twisted, father-of-the-bride slash husband-of-the-bride type territorial way, I was like, “umm.....
!”
If my bf was ok with my fucking other dudes willy nilly and all up in his face, or vice versa for that matter, something would have to be horribly fucking wrong.
At the same, at the risk of contradicting myself, lifelong monogamy, especially in its potentially decades long capacity, is also unsustainable for a lot of us.
What a hard circle to square, huh.
The only flimsy, possibly disastrous/potentially workable compromise I can come up with is blanket permission for safe flings between the two parties without their knowing the who, what, where, or even if of it.
But, honestly speaking, it seems the only way that doesn’t signal an unhealthy dynamic and a personal lack of regard (low self-esteem) is good old fashioned covert cheating the other party isn’t aware of, which of course, reliably signals that you’re a dick. How can you win.