Does anyone know of any good study linking ADHD to underlying physical condition, like chronic inflammation or coeliac?

My kid has some form of behavioral disorder but it doesn't really check all the boxes of any known condition. He doesn't have attention problems. He's very active, but not unmanageable. I don't think he's autistic, though he's socially awkward. He's very defiant with adults, so OCD does match (though that doesn't help much). Definitely very low self-esteem and all that. Mostly he's aggressive, low patience, disobedient AF, requires a ton of attention.

A particularly telling thing is that when we take long drives we often listen to children stories. He can listen for them for hours no problem. But when a story stops and until another one starts, he gets really nervous, makes a bunch of noise, annoys his brother, like he can't be canalised if he's not focused.

But what strikes me is that he seems to have a bunch of other conditions that doctors dismiss as benign. He has low teeth mineralization, often small skin bumps around his joints. He has a geographic tongue. He had digestive track pain when he was smaller. He had discoloration on his cheeks. He is quite skinny while he eats normally.

ChatGPT told us he could have coeliac, but we blood tested for coeliac and it came back negative. We tried a gluten free diet for a bit anyway, and that didn't seem to change anything at all.

We're like very puzzled. It's pretty clear that a psy test will find something behavioral, but no one is going to look at any kind of mineral deficiency, or anyhting like that.

Posting this here, Idk why, I'm kind of clueless about what to do at this stage.