Does he even deserve an explanation.
Nope.
He's not interested in a relationship with you, for whatever reasons (it may be he's not really interested in a relationship with anyone .. or even if he is, it's not you at this time and may never be) .. happens to the best of us. Every one if us has been rejected or not chosen including famous women who are envied for their looks etc.
Tbh his son having to tell him he thinks he should settle down - sounds like an ol jack the lad, wannabe player, fancies himself "free spirit" (wanker,) type.
Maybe he wants a (much) younger woman, maybe he wants a woman with serious assets,maybe he wants a woman whose kids are at a certain age, maybe something specific looks wise is his desk breaker that you don't happen to have (I dated an older man who liked big boobs and i didn't have them, he told me everytime he was tipsy, unsurprisingly the relationship didn't last), maybe he likes certain personality traits that aren't really you ..
Who the fk knows.
It doesn't matter, he's not offering you a relationship, you're feelings are getting hurt, it's unlikely to change. Concentrate on meeting other men not like him, on having fun and building up a great varied life try some new stuff.