The father trains his son to last as long as possible before cumming, so when he will fuck a pussy for the first time he won't make a bad impression. This training is probably done every day. The son must try as hard as possible to hold back the cum, otherwise he will be punished.
I used to visit an older guy when I first started out. I was 17, was in his fifties. His was the first cock I took in my mouth. I loved sucking him to completion. In return he would wank me off. Exactly like this. Fore finger and thumb. I would squirt just like this guy. He loved making me come.