I could see him being purebred, actually. He looks a lot like the older TN Walkers I've encountered (online; they're exceedingly rare in Aus).
As an unrelated aside, have you thought about having some bodywork done on him? I have seen some incredibly gentle therapies do amazing things with older horses' toplines. "The skinny one" at my farm (a 22 year old TB who has looked like he needed rescuing for YEARS) showed absolutely stunning improvement with regular Emmett therapy :)
MAKORA THOROUGHBRED SPORTHORSES