I don't see a sway back at all. His head is up on both photos, so back will be more down. He is under muscled, but his overal conformation is actually good. Feet need to be trimmed and maybe have farrier work the hoof back so he has a bit more heel. Mac (the appy I lease) is the same way ; heel is low and hoof angle flat. He is, however, extremely hardy.
I actually think that that horse, muscled up and grooomed, will be rather pretty.
I will admit that his neck is a bit short and thick. Kind of ponyish.