have the vet check his iron, if he is low then he will mostly have you put him on redcell
My barn has a couple horses on red cell for this sort of thing, it seems to help them.
“Lazy” can be EPSM/PPSM. Might be worth the $65 for a genetic hair test, especially if the horse is a draft cross.
Having dealt with versions of this (everyone else thought it was just being stubborn/training issues… despite being in training with a pro), I can tell you that ALCAR and a diet change has made a huge difference. I have a different horse now. 
Have your vet check your horse’s thyroid function. If it’s low no supplement except for Thyrol-L will help.
I wouldn’t use Redcell or the like until I had the thyroid checked and the blood checked. Most horses get plenty of iron in their diets but cannot utilize it because of a copper deficiency. When you add copper to the diet the iron count rises dramatically–that’s why Redcell and the ilk appear to work.
Too much iron is not good tor a horse and can cause a whole other host of problems. If you use an iron supplement only use it for two or three weeks, or you’re headed for trouble.
A scoop of powdered caffeine in their feed does wonders. Give it an hour or so before they work, so you catch them coming down - makes them less fighty.
It can also be mixed with something like Wind-eaze, if you decrease it gradually, eventually you get the same reaction without the caffeine (and it doesn’t test).
