I heard a statistic the other day that 1 in 4 American kids are raised by a single mom. ReverendRuby said that your son will see all of his friends with their dads and want the same relationship, but the fact is, there are many different kinds of families these days.
My daughter met her biological father when she was about 3, and hasn't seen him since. I prepped her by telling her there were many different kinds of families and family relationships. It helped that we have a diverse set of friends; we are friends with a gay couple raising a couple of boys, and a couple of single moms, and a single mom raising children with a live-in boy-friend. I told my daughter that everyone has a biological father, which isn't always the same thing as a dad. I told her that a "dad" is a person who has a very specific relationship with a child, and that's something that has to be developed over time. Over the years, I told my daughter that I would get married some day to a good man who would be a dad for her, and I told her that her biological father had some personal problems and couldn't visit often or be a dad at that time. I always referred to him as her "biological father" or by his first name. We did run into the problem of my daughter wanting to call a boy-friend of mine "dad", but I made it clear to her that the relationship had to be there first, and the guy in question had to be ready and willing to have that relationship, and I insisted that we had to be married first. (Reminding her that all along I'd told her I would marry the guy who would be her dad.)
I had made it clear to my daughter's biological father before they met that he was not going to be my daughter's "dad" until he developed that relationship. He wasn't thrilled about it, and he would have preferred to introduce himself as dad, but he never tried to develop the relationship. It's true that he had some "personal problems", but that same explanation can be used in a lot of situations; some people are pathologically selfish or immature, and that could realistically be described as a personal problem that doesn't require further explanation.
Tell your ex how you feel, but remember that regardless of how your ex introduces himself, your son will accept YOUR definition of the relationship over some stranger's.
HTH, good luck :)