I used to suffer from the same condition. It is known as delayed ejaculation or retarded ejaculation. There are a few possible issues. One could be low testosterone. Leave that one as the last resource, as bringing it up and asking him to get tested for it could lead to a further dissatisfaction in the bedroom. He would likely shut down and not want to have sex period if he thought that you felt he needed a doctor.
But two other things that have worked. One: sex is a different feeling than masturbation, and the friction is not the same. If he is willing to forego masturbation, and you are willing to have sex with him a lot, that allows him to feel the friction of sex more.
Two: his thoughts. He has to think of things that turn him on. Perhaps put pornography on (of his choosing) and let him know that whatever types of fantasies he has and wants to watch is ok. There are a lot of fetishes, and even if they are ones that would never be acted upon, the fantasy is still a turn on. So you would have to not judge him, regardless of what type of porn that might be. Getting him to think of fantasies and things that turn him on helps a lot.
If you could use the two together, it should work. Ask him if he would be ok not masturbating, but having a lot of sex. Also see if he would be ok with pornography while having sex, with no judgment. If you do both of these things, you should see a change within two to three weeks.
If this fails, then testing for low testosterone is the last option. Also, if he wants to stop having sex, DO NOT FORCE HIM to keep going until he finishes.