Well, the only way to do it is talking!
No 'pointing fingers' or 'throwing indirect messages'. Sit down and talk! I know you've been doing this, since you already got on doing the thee way. Maybe the reason your wife doesn't want you to do oral to him is because, uh, you're both guys? Well, I mean, talk to her and explain your situation. Tell her what you feel and the reasons you want to do it and if things are going as great as they are now, she wil understand and maybe she will let you do him sometimes. It is just all about variety: Imagine the thousand of different things a couple can do, a three way, there's millions!
I wish you good luck, my friend, and a happy sex life too.
-Jean.