Please try out different setting for your sex session.
Have your session after a good night rest, to rejuvenated oneself before the vigorous thing.

Good or healthy sex, do not always have to depend on your partner's performance. You can self-help or self satisfying, relegate him to being your side-dish, with the intention to alleviate his pressure to perform whereby, both may learn to enjoy the session more.
If your situation didn't improve, ultimately, it is best that he seek professional help when he is ready.

Side note:
Inadvertently you may have piled more pressure on your bf since you have openly discussed it. Probably, masturbation is an escape route for him, with no expectation to hold it up and putting his virility to questioned.