Or it may be he's a person who just isn't comfortable participating in a poly relationship. There may not be anything anyone can do or say to convince him. A very good friend of mine, who knew my husband, knew the situation, everything, still couldn't wrap his head around he and I being together. To him, in his words, it was, "just too weird!"
He's still my friend and he doesn't judge (most of) my lifestyle decisions, but his choice for himself was to not be romantically or sexually involved with me. Made me very sad and angry and disappointed at the time and it took me a while to get over him in that way, but that's the way life goes sometimes.
Pan Female, Hinge in a V between my mono (straight) husband, Monochrome and my poly (pan) partner, ThatGuyInBlack