Sorry, should read,
the more he is inside you should help.
OK you could try with him half in you so he can masterbate the last bit, doggy style should work for this, or just some long slow sex with you on top.
But your frequency would help him overcome this, 4/5 times a week, because the more he is inside youhould help, and anytime off spend it in bed making love, I think its a case of getting him hard and keeping him inside you for as long as possable, and a case the more hes doing this that he should overcome it with time.
Do you know that if as a young man that he masterbated a lot, if so its a case of what he did with his hand is remebered in the brain.
Hope this gives you some help.
Good Luck
I have experienced that problem. But I don't worry about it. Maybe introduce something exciting while you are screwing.....turn on a porno, put on some piece of clothing or footwear that turns him on.