Why would talking to her son make you want his mother back? The two don't even go together to make a correlation in my books. Either you are the young mans friend or you simply want to be in touch with him to get to his mother.. which one is it? Further: You forget that his mother in a relationship with the man she got with after she dumped you. That should be enough for you to hitch up your britches and find your own girl (your own age because chances are it will work out better the more you have in common with one another).
If you are unable to keep this young man as a simple friend without linking your thoughts of him to his mother then do both you and him a favor and grow up and leave them both alone. Divorced couples with children have to learn to live without wanting their children's mother/father so you can do it too.
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion