While your issues with this horse may certainly be behavioral/learned, you didn’t mention how old he is, or his previous training? Was he ridden or perhaps used more as a harness horse, which might account for his ignoring your signals? If he was trained primarily to drive he may have no idea what leg aides mean and needs your help to find the right answer.
My experience as a barefoot trimmer certainly suggests that so-called “lazy” horses are often in some sort of pain. It may be that more than one foot or leg is involved, such that no lameness is obvious. Sore feet, chronically cramped muscles, back problems or arthritis from previous injuries, etc. may be the cause. How does this guy behave when he’s turned out with other horses on a cool and exhilarating day? If he just mopes around and prefers to walk everywhere, I’d certainly suspect some physical discomfort, unless he’s quite old. I’ve seen huge draft horses running and playing–they literally shake the ground at the canter, though I get cobs are much smaller in size (but would expect a somewhat similar temperament in terms of “ambition”).