Hi, welcome to HCM. Thanks for asking your query at this forum.
We understand your concern. I have read your question carefully and understood it. I will try my best to answer it.
As you told, your boyfriend is having increased desire for sex and he frequently masturbates to get satisfied his urges. Sex depends upon the state of mind and body. After ejaculating once it generally satisfies the need for some time and it may take a period of time to get prepared for another sexual act. Similarly orgasm is also delayed/absent during successive sexual acts. I can understand how painful it may be for a female partner.
It may be side effect of some medicines also (like
SSRI group of medicines if he takes), but it is unlikely in his case as you have mentioned nothing about his medications.
In my opinion if he reduces the frequency of sexual act, the problem can be solved. You may ask him to indulge in other pleasurable activities like games and sports, watching favourite TV shows, listening favourite music etc.
If he can not reduce the frequency then you should have first sex with him in first half of a day then later on he may engage in
masturbation.
If you both feel that his sexual urge should be controlled then he may need to be prescribed few medications/
psychotherapy for some time.
I hope it answers your query. I wish you both all the best.