First off, your body is yours regardless of who it does and doesn't turn on. There are lots of people who would probably love your body. The fact that you "aren't fat enough" for him is a problem with him and not you.

Second, don't gain weight for him. Do not change your body stop try and please someone else. You deserve to have someone recognize and cherish the body you have, not try to make you lose or gain weight. If you are losing or gaining weight to satisfy a partner, that's a good indication that you are being emotionally abused.

Thirdly, if he's serious about fixing this, he needs professional help to try and combat this because having a fetish for a certain body type to the point where you can't get off without that particular type is bad news for you or anyone he dates. But do NOT change yourself for this guy or anyone but you.