His behaviour sounds odd to me - one of mine changed behaviour in an instant last month. I am sure it is magnesium deficiency. Google it for lots of info. Good Luck.
My trainer always tells me that our horses are sent to teach us. I truely belive she is right.
I've had many teachers in my life, but the best ones have been horses.
Wishing you the very best of luck, enjoy the journey you are going through with your horse.