I am so sorry that you are going through this and as a PP has already said, that awful feeling.. I know exactly how you are feeling right now and wanted to send you a big hug.
I haven't read all the replies you have got so apologies if I am repeating what you have already been told but the first thing I would be doing is sitting him down to talk it through and get some answers, calmly, despite how you might be feeling.
A calm approach is likely to get more answers than going in all guns blazing. A good way of broaching it is to infer that you already know what 'might' have gone on (even if you don't fully know for certain).
For example.
'We need to talk and I need to ask you some questions. And I just want to preface these questions by asking you to be completely honest with me. Regardless of you worrying about what you might say hurting my feelings - I need you to be 100% truthful, and before you answer, I want you to think very carefully, because there is a very strong possibility that I already know the answer to the question you are answering and by lying you will only make the situation worse. I am aware that something has happened on the stag do and I want you to tell me, in your words, what has gone on'.
I have found this to be quite an effective way of getting the truth out of people. Not saying that this will work, but worth a try.
I know it might feel like the end of the world now but until you actually talk this through and get some solid answers, you shouldn't be making any hasty decisions. You know him and in your heart, you will know if he has done this or not. Lean on your friends and family for support and you mustn't worry about calling the wedding off if this is what you need to do. It is far easier to cancel things now and lose deposits etc than go through with it and face the consequences on the other side. Don't make a mistake you will live to regret.
I want to send you lots of love and strength to get through this whatever the outcome xx