Like most people have said, I think it is a bad idea since you are his boss. Eventually all your employees will know what is going on and may lose their respect for you. Business will go downhill from there. Not worth it.
That said, you might tell him that if he finds a different job, moving in is a possibility. Make sure he gets established at this new job for a while before working out the details of moving in.
He should pay rent, period. I think he should also have his own bedroom, even if he doesn't ...... in there much, so that when friends or family who don't know of your arrangement come over, it just looks like you are renting out a room to this man.
Figure out in advance all the usual stuff as if screening a roommate. Does he need to buy his own food, and fix his own meals? Is he a slob that is going to leave messes for you to clean up? Will he help pay utility bills or does his rent cover everything? It may turn out that while this guy is a great bull, he is potentially a terrible roommate.
Yes, he is the bull and he is fucking your wife, but he needs to know that if he makes life at home inconvenient for you, or your wife gets bored with him, he will need to find another place to live, right away.