I'd say if you feel unable to talk to him about it, that's probably a sign you shouldn't bother. Have a good 3way with him anyway, and you should be able to sense whether he's open. If he isn't, have fun, than go out and someone who is down for it.

In my experience it’s better not to ask, just arrange the threesome and on the night have a few drinks and relax, at some point he’ll be fucking your wife and you can be kissing her or just watching him slide in and out, when the right moment comes just start sucking on him, if he doesn’t like it he most probably ask you to stop and continue fucking your wife... for me each time I’ve done this the guys loved it

Well that sounds amazing to me! I’d be a guy that would participate in that.

Where do you live? My wife has exactly the same fantasies.

You may not want to ask him for that. If he is straight. Then you can find a handsome bi guy to do it with. Just keep looking and you might find someone who is totally willing to fulfill your fantasy.