I don't know much about ADFs or anything about disease but I figured I'd put in my 2 cents anyway. He looks really thin from those pics, not sure if I'm just used to my ACFs though...

Maybe a stupid thought, but is there any chance it's simply a full belly? Is it there and prominent all the time? I know after I feed my ACFs I can see a bumpy bulge in their left side, where their stomach is. Goes away by the next day though. If he is underweight it might look more prominent than on my guys.

Not too much help, but I really hope he's not sick and someone with more knowledge than me comes along soon!