He looks like a Walker/QH cross to me. Gaited horse crosses often have the nicest trots, along with laid back shoulders and higher withers.
Or, he could be a TB/QH (Appendix) that just didn't come out very tall. My bet would be walker/qh though. Or standardbred or saddlebred cross.
He looks better when he's thinner too
. He's gotten cresty and goose-rumped with all that weight on him. Pictures 4 & 5 show him at a good weight for his build/type.