To prove he was the real deal...
All he had to do was give a simple DNA paternity test to see if he was actually the father of the psychologist. There's no way he could look 40 and still be a father to a man clearly much older then him!
Also, Someone could have snuck hair samples without him knowing? There's also many samples they could have gotten from the restroom or left over food! Even a finger print would have been enough! Once they had that, they would be able to track him for years to see if he actually ages!
This is supposed to be a movie with realistic idea's, so why didn't anyone try it? In real life, surely that would have played out?
I personally think, he's a man running away from his responsibilities, who owes a substantial amount in child support! He must have a lot of children from different women he's not up to committing! Just my 2cents...