Cat is super skinny

My cat turns 2 this month and for the past year now he's been incredibly skinny. My mom and I have been to the vet countless times and they've run blood tests but keep telling us that they don't see an issue. The visits are incredibly expensive and they want to do an ultrasound for about $400. My mom had already spent nearly a grand trying to get him help and nothing is working.

He used to have very runny stool and would constantly throw up but he hasn't done that in a while. We try to keep his food dish food all day so he can eat freely but we just don't know what to do anymore.