Can people please stop trying to derail the thread talking about advice being different if its a man. This thread is about getting the OP some advice.
Personally OP, I think that from what you've told us you should leave him, and I don't even think you should bother to give him any last chances, as he's made it clear he can't be bothered to follow through when he's made promises previously, so why waste any more time on him?
It would be different if it was a health problem, or something like that, that has caused him to go off sex, but the fact that he's actually masturbating while in bed beside you must feel humiliating and like a real slap in the face! Also, the fact that he's masturbating rather than having sex with you, makes me think that he really is fat and LAZY! After all, let's face it, unless you do all the work, making love takes physical effort, and it seems to me, like it just feels like too much hard work as far as he's concerned, so he just has a wank to relieve himself instead, with no thought whatsoever about satisfying your needs.
So, in MN parlance, get your ducks in a row, pack your bags, and as you walk out the door just tell him that he's had all the chances he's getting, you need someone who WANTS to be close and loving with you. He clearly doesn't want this, otherwise he'd have made the effort to sort out his weight, and get fit enough to make love to you, so you're off to find someone else. Don't listen to any arguments he comes up with, just walk out the door. It may be the wake up call he needs, and you might find in a few months that he comes to find you, looking like a completely different person, but I somehow don't think so, and if he does, well, it won't do you any harm to have the opportunity to explore what else is out there in the meantime, will it?