If it turns out not to be diet or pain and bloodwork turns up nothing, don't forget to have his eyes checked. A change in vision to a nervous horse could easily cause an explosion.
I have a lesson gelding that went through something similar a couple years ago and became so spooky and hard to handle that we couldn't safely ride him or use him around the kids at all. We tried a variety of things, had the vet test for several things, check eyes, back, etc. We never did find a solution that panned out. This went on for about 6 months and then it was like it never happened, he did a complete 180 and became the same calm quiet boy he'd always been. I started him over in his training at that point just to be on the safe side, but never had another problem with him. It was one of the most baffling things I've ever been through with one of my horses. I still wish I knew the answer, in case it ever recurs.
"Keep a leg on each side and your mind in the middle"