Much of this is similar to ds1.
He had undiagnosed tongue tie until I figured it out when he was almost 6yo.
It caused him problems with chewing and swallowing. He refused pretty much everything unless it was soft or sloppy. He gagged on certain foods, stringy stuff like sweet potato for example because he did not have full control of it moving around his mouth.
He drank loads of liquid. I later realised when he was able to verbalise burning in his chest and being sick in his mouth a lot that he was drinking to relieve the burning.
His stomach WAS huge after eating anything that needed a lot of chewing because he was swallowing down air when he ate. I remember him having a distended stomach as a baby after feeding, but I had no idea why. When he started doing swimming lessons, he was conscious that his tummy was bigger than most of the other kids' and he wore a rash vest in the pool to cover it.
He would take a few bites then say he was full. He was full. Full of air, not food.
Meal times etc would have been a drama if I had not let him eat the things he was happy to have. He was really good at trying new things and often, he would put it in his mouth and say mmmm, but then when he tried to chew and swallow it, he would then say he didn't like it.
You mention that your ds has an unrelated health problem. Tongue ties can and do cause all sorts of problems that you would never believe were related, such as ENT problems, sleep problems, orthodontic problems, mouth breathing to name a few. Obviously I have no idea what your ds's other problem is, but mine had a whole host of things going on. It really affected his overall health and his whole body. There are pictures of his ties and high palate on my profile.
How did milk feeding and weaning go?