If you weren't with him at the time what can he truly say? I would understand if you were with him and did this that he will be upset but you weren't.
If it is completely out of character for you to do what you did back then, his first response may be that of shock. But if he is loving and supportive he will soon get over that and realise how much you must have trusted him to come clean in the first place.
I would emphasise my point to you that I would only tell him Bold:if you thought it would help and be useful. If you feel telling him would make matters worse then no need to mention it to him, it doesn't effect him.
It sounds like you really need a good friend to talk to, which may and may not be your husband. You need to deal with the guilt of this somehow and let this go.
We have all done silly stuff when we were younger especially when it comes to the opposite sex. It's all part of learning and growing up. You made a mistake. Please don't think you have to spend the rest of your life paying the price for it.
Maybe writing a letter to your dh, the older bloke, or even yourself but never sending it, but burn it instead will be a cathartic release for you? You were young and whichever way you look at it, you can in no way accept all the blame for the fall out. It took two to tango