I think he is just a stocky appy!
he of course has the markings for a leapord spotted Appaloosa (but you of course knew that:).
He also is like you said very stocky, and draft like! Which leads me to believe somewhere back in his bloodlines he could have had some form of a draft ancestor, but I don't think it was any direct relatives !
Or he could just be a pure appy! With a lot of muscle and a little bit chubby ;)