My Arab is very lazy. He is 26+ yrs now and still a very lazy horse. He is so lazy coming in from pasture at night that Mr. WTW calls him "The Stoner Horse".
He had an injured vertebra when I rescued him 19+ years ago. So injured that he's seen a chiropractor more than I have
He now has a nice arthritis bump in the injured area that bothers him.
Is he sandbagging to some degree? I'm sure he is but he's always been extremely well mannered, a great lesson horse for children, leaves them with happy horse memories, so he can sand bag all he wants.
If your horse has not been checked, structurally, I would have a chiro look at him. Anything is possible; horses don't have to be lame or look/act different to have something wrong them with. He may still be lazy after treatments (if he would need them) as it's become a learned habit but at least the real reason for his laziness would be known. He could also be uncomfortable internally, when he breaks out of a trot or canter but he's too much of a gentleman to buck about it
Point being, were he mine, I would make sure he is healthy before I pushed the riding crop too much - doesn't mean he doesn't need it just make sure there isn't a legitimate reason behind his lazy behavior