I, too, disagree with that technique. But be that as it may, I just wanted to remind you that slowing the lope is very hard and strenuous for a horse that is used to using momentum to keep going. Be sure to give him lots of breaks and breathers. You are stressing hundreds of small tendons and ligaments, as well as hocks and stifles to get the slow lope.
Collection isn’t a mindset, it’s a physical ability. So far, I haven’t found any shortcuts. It has to be systematic building of strength in the athlete.
20 foot circles at the lope will tear up his hocks…much too much stress and pressure on the hock joint to be working at such severe angles. Or you can continue as you are and in a year or two start pouring thousands of $$ into vet bills for hock injections and supplements.
Your horse is not being stubborn. You are being an inconsiderate trainer.