Sounds like they are a perfect match.
You say you "dated him" then claim it was a relationship. Then he just up and left and never contacted you again.
His son's girlfriend's Mother and him have known each other for at least 5 years and now they have realised they like each other, probably through conversation of their children getting married.
Yay, he's finally found someone that he feels comfortable with, maybe he has not taken himself off a dating site yet as he wants to see where this leads first, that is what any person would do.
Don't be bitter. You weren't suited, simple.
But, I am also betting that this has occurred to you more than once from a dating site.
Try mingling in group situations of things that interest you. Maybe you will find someone worth while that will hang around.