I don't think there is a winding way that you can ask the question. You can just tell him that you have some important to discuss and go straight to the point. But you shouldn't attack him or something like that. You just have to let him know that you are concerned about it .
I don't think you are overreacting. After all, you will just be talking with him, right?
As far as I know, it's not a regular occurance for straight guys to talk about gay sex in their sleep, just as it's unheard of for a gay guy to talk about women in his sleep.
And yeah, it's possible that didn't cheat on you. That he was just having an unusual wet dream about performing oral sex. It's also probable that he did.
Like I said, there's reason for you to talk with him. For everyone's sake. You can always talk to us or anybody you feel comfortable with.
Take care, good luck. *hug*