Don't ever try medicine or whatever supplement or solutions, the answer is simple. He's beginner, instead of carrying him to masturbate, why don't you have sex with him every single day. Let's him learn how his man made work.

If his pen not comes straight, give him a rest. Look upon each other, try to talk longer when you have time with him. Build an intimate talk or even normal ones. And last but not least, play with his pen always.

Role play is a must to satisfaction.