I used to think I had relatively low sex drive, and masterbated out of habit...

I stopped masterbating about a year ago, and the difference in my sexual outlook, energy, and positive happy health is massive.. Excessive ejaculation depletes so many vital vitamins, testosterone, serotonin, dopamine... your body has to work very hard to produce semen, and excess masterbation and excess ejaculation will definitly cause a low sex drive...

But the reason he will still masterbate despite this apparent low sex drive is it is a kind of low level addiction, in which facets of fantasy or porn replace the living reality of the actual delicious squishy flesh of partner, which become desensitised...

If you see a female body with full living vibrancy, you see that person as a goddess you want to serve and caress and massage and lick, and have endless foreplay and deep sensual sex... nothing is of more interest than giving the female a good time, and approaching it like that leads to divine experiences, well beyond the usual cum cycle...

but yes, if the senses are deadened, it's like wearing glasses that make everything appear cardboard... so it's like your body is overlooked for the amazing wonder that it is...

The secondary problem is that if sex with you always ends in him coming, and you did it 5 days a week, this would also diminish the sex drive, which is why it may become, easier to just choose to masterbated instead...

That's why tantric sex exists, in which you can have your cake, and eat it, so to speak.. but isn't easy as it takes a lot of unraveling of all the sexual conditioning and unconcious patterns...

The start point here is communication, give him a fair chance and see if you can help him to see what is happening.. as I don't think this situation is sustainable or something you should have to put up with, and if it is impossible to get anywhere with it after fair effort and giving him enough opportunity, it's the wrong relationship... you deserve more..