Thank you for your input, dear friends. I've decided what I going to do about this: nothing.
At least not for the time being. His feelings and whatever issues he may be experiencing are not my fault, my creation or my responsibility. The fact that I know one valid solution to his dilemma concerning his attraction to a married woman does not obligate me in any way. And the same God who lead me to polyamory can lead him too, and do a better job of it. And if I open that can of worms with him, I create a problem for myself, potentially a gigantic hairy monster of a problem that devours the city, or at least certainly ends with somebody getting hurt.
If he were ever to ask me a direct question, I'll answer truthfully and directly, same as I would for anyone else.
As for his habit of speaking over people, and the other similar behaviors, it's not for me to fix him. I have a full time job trying to fix myself!
If he ever seeks my advice, I'll give it.
Thanks for the help, everyone. I sincerely appreciate it. Sometimes the wise, compassionate and loving thing to do is what you're already doing.