If you're serious regarding this then request that he start wearing a jock strap. That'll help tremendously ... however, I'm betting that's not the message he's wanting to send to you & your wife.
So, next time it happens (with your wife present) just tell him if he's going to flaunt his cock around your wife to just take his pants off so she can get her eyes full ... that'll break it into the open, and she'll be fucking him within the hour. Your wife's not getting the quantity or quality of cock that she wants, you know that, he knows that, and she most definitely knows that. So rather than encourage cheating, get it all out front ... at least you'll be there to watch it AND probably do your "cuckly duties" after he's done.
Everyone will be so much happier ... Mac