I would confront him with the rumours you have heard.

Say something like "Thought you might like to know, etc.... How you were shocked that anyone could even contemplate spreading those rumours considering how HAPPY he has been saying he is to be getting married ........ perhaps it stems from him being a "touchy feely" kind of guy, and people got the wrong idea????
(Not sure if he is a touchy, feely type of guy??)

If he still denies it, I would refer to EXACTLY what you have heard .......... and ask him if he is absolutely certain this is what he wants to do before he gets married ...........

After that, there really is nothing you can do, but try and make very clear to him that his fiance would probably want to know the truth, and it would be up to him to tell her .......

Gee, I hope it works out OK ...