I assume your from the U.K right?

Tricky situation. However, I don't think it is wise to follow through. Hes 20, and at that age us men just want to get laid. Getting with someone who is 10 years hes senior who mind you ALSO would be carrying baggage (The child) is alot of responsibility. Responsibility I believe a 20 yr old would not be able to handle. Cut your losses. If your that unhappy in your marriage I suggest you leave your husband.