Scarleteen

my gender fluid partner (at the moment' he's a he) says i'm gorgeous but thinks he's ugly, partially stemming from his weight, which is normal but he got bullied for people thinking he was fat in the past, how can I help him? I try and tell him how much I think he's pretty but nothing seems to work

It's great that you want to support your partner in this! It can be really sad and stressful to feel that a friend or partner's body is wonderful while they're feeling so negative about it. Ultimately you can't change his mind about this so I don't think that needs to be a goal, but trying to be positive and affirming is a good thing. I think when he says something negative it's best to say something like "I'm sorry you feel this way, I feel X way about you" vs. "no you're not" so that you aren't minimizing how he's feeling in that moment but showing you feel differently. It's also ok to ask him to cut down on negative self-talk that he does when he's around you.
A suggestion for him might be to really dig into fat-positive resources & blogs and spend some time looking at people of all sorts of body types and finding what's beautiful or attractive about them. Improving self-esteem and body image is a huge undertaking but I think surrounding oneself with the thoughts and images of people who've made great strides in that area is a big help.

I know this isn't a big deal but I'll just say this, he isn't actually fat. he's not obese. but he was harassed and bullied into thinking he was to the point where it threatened his well being, some time in the past. I don't know much of the details and I'd rather not ask him because it hasn't come up in conversation again and I'd rather not bring back those memories for him

I don't think someone has to actually be fat to have internalized a lot of fatphobia, especially if they have a history of harassment around their body. Unlearning that internalized fatphobia and finding positive aspects of bodies of all sizes is super helpful to people of all sizes, too.