…just read you only had the horse 3 months. Did you ever do x-rays of his knees & ankles? That’s what I would be guessing now. I had a mare a few years ago like this that “checked” out by 4 vets as sound on the flexions and jogging. Mare continued lugging out & didn’t feel bad galloping. Had her x-rayed & what she had was a chip in the knee that was bad enough to affect her while going a decent gallop/breezing/running, but not slower than that. She would jog/gallop undersaddle sound as could be.
Just a thought. I had to ship mine to the clinic for x-rays as the vets on the grounds said she was fine… Had another one too that took two people to walk in the shedrow as he was such a good-feeling horse. Checked out by 2 vets as being ok. Legs ice cold. Jogged with the rider ok…galloped sore. Had the vets scratching their heads. Took him to a clinic & he had a fracture of the cannon that we needed the better x-ray machine for (he WAS x-rayed all 4’s at the track & nothing was found).
#1. Check the teeth & if they ARE bad it might take a week or two for the horse to ‘realize’ it doesn’t hurt & changing to harsher bits will probably just make the problem worse…
#2. Take x-rays of front legs.
A friend at a neighboring track witnessed last month a gelding that lugged out so bad he ended up one morning – just galloping – going over the outside rail injuring a VERY good exercise boy. Horse had just win 3 in a row & was also a speed horse. Also had chips. Horse ended up ok out of the incident (rider unfortunately not as lucky) but I don’t know who’s going to want to ride him for the races now…
Maybe you have the ‘fluke’ that it’s a taught bad habit…but for me I would do everything possible to find the problem before just changing bit after bit ‘hoping’ he stops…