I still have many questions to ask, but will try to give you some tips anyway.
Have you had a dentist who specializes in dentistry do his teeth?
Have you had a vet examine his back and right hind?
Do you know a chiropractor that could take a look, as he may have something out?
How does your saddle fit him? I would use a really good pad.
It is hard to tell, but it looks like a normal snaffle with no flash (if he has a flash try taking it off). Is the nose band high enough? I like them just ½ inch under the cheek bone but not too tight. If it is a single jointed snaffle, try a french link and vice versa. Try a rubber D single jointed snaffle to downsize since he resists “contact”.
For you as the rider: go back to basics with him. Try a looser rein, with much lighter contact. No clashing aids. Perhaps try holding the reins with a driving rein to lighten your hands for awhile. Don’t ask him to be on the bit now, just be happy with light contact or semi-loose reins. He doesn’t want to go forward, yes, but you are legging him constantly in the trot. Leg, leg, leg, every stride. This will definitely annoy him and make him dull and non responsive to your leg. When your leg closes and asks him to go forward, decrease your contact. You must give. Right now it looks as if it’s gas and breaks often at the same time.
He must learn to go forward on his own with the slightest leg. If he ignores your leg, come in with a kick, or little tap with the whip, and loosen the reins. Keep your heels down, and don’t keep legging. It’s consistent corrections to ignoring your leg that normally solves this.
Your position at the trot is too upright. Keep your upper body angulation, or hip angle, more close to approximately 30 degrees to be in balance with him and encourage impulsion. Post more toward the pommel, and sit lighter in the saddle.
Learn to follow his neck movement at the walk with your arms (following arms in the walk, canter, and gallop). You want to develop elastic arms that move with the neck and not ride at the walk, or canter with fixed hands.
Thank you for this post. I have a similar issue with my 9 year old OTTB, primarily when flatting and in the canter. He gets regular chiro, dentals, etc. and the saddle fits. He’ll get choppy and reluctant to go forward. He anticipates me tickling him with the whip and as soon as I touch him, he kicks out. After that, he’s fine. I believe it’s just laziness, but what are some tools to break the habit?