5 year old afraid to eat

My 5 year old is afraid to eat. He has hyperoxaluria, so we keep a low oxalate diet for him so he doesn't get kidney stones. For a while he was afraid of food because of the pain of the calcium oxalate crystals. That's more under control now and has been for a while, like a few years. But recently we all had norovirus and a lot of GI issues, and now he's afraid of food again. It's like all the progress we made with eating has been erased.

But it is not just potential stomach pain he's afraid of. He's expressed he's afraid of becoming fat. It's hard to talk about emotional things with him because he goes mute and starts signing, but it's mostly a sign language he made up (we're trying to learn real sign language together to make this easier for when he finds talking hard, but that's a whole 'nother thing) so it turns into a guessing game. I wish we could just have a regular conversation.

I don't know where he got the fear of becoming fat. We're very conscious to never refer to food as "bad" or "good" and just talk about nutrition, and that everything is good in moderation-my sister in law was in inpatient treatment for ED before he was born and it's something we take very seriously. I'm so scared he's developing an eating disorder.

It's a fight to get more than 500 calories in him a day. We've taken him to the hospital for IVs. We're getting him started in feeding therapy this week. He told me tonight he felt like I didn't love him, because I made him eat a spoonful of peanut butter for the protein. I wound up having to feed it to him one fingerful at a time.

Meals are so hard. He's not that picky taste wise, though he has some sensory aversions and doesn't like spice. But he is either in the mood for something or he isn't, and I can't just say "fine it'll be here when you're hungry" because he'd just starve himself. He doesn't find eating enjoyable.

I am scared. I want him to have energy to run around with his friends. I want him to have energy to grow. I am so frustrated. I feel guilty for times my temper has gotten short trying to get him to eat.