Just be open with him when he is super sexual charged and brings it up, go with it and ask him how he specifically wants it to go down/happen. Then when the moment is over casually bring the subject up again.
With me I would bring the howife subject up while fucking my wife. During this " super sexually changed" moment I would ask her while fucking her how would she like a big new cock to fuck. At first she would say I was all she needed, but as the years went on she started to say yes I would love to have new cock/ a boyfriend that can fuck me anytime I need it.
I would have to admit when she started responding like that I was feeling like wow she really wants this and started to feel a little worried and uneasy. However, the worried/uneasy feeling was not greater than the excitement or passion I had hearing her respond in that way. Also, her pussy would get so wet I knew she was getting extremely turned on.
What happen next is she started to ask me after sex if I was serious. She stated to take advantage of the situation (after we were done having sex) and would stroke my cock and ask me specific guestions about: who, what, were and when I would want to have this happen.
She really kind of forse my hand on the subject, but like I said I was more excited than worried/uneasy. So I was just honest with her that I was very turned on by her being sexually pleasured by another man.
At that point we had very open discussions and she came up with a few rules: never at home, never with men we know, she decides with who and when and a threesome with me and another guy was not on the table yet (she felt like she wanted to explore on her own first.)