He is proberly fine but check with the vet just in case! here are some things that could be the problem:

1: he might have had a traumatic moment like a dog attack or bad fall ect

2: He might have something wrong with his tack, dirty tack can be dangerous, you need to clean more than just the seat. 

3: he could have fleas or worms.

4: he may have a back flinch, in which case, you should call a vet! 

Good luck, i hope he gets better soon! 

Is your horse out in a pasture all the time? It could be coyotes! My mare was like that too. I've had her for a long time and she was kinda spooky because they coyotes where out at night. What I do is ride her with a buddy so, she feels more relaxed.

Has anything changed in his routine? Change of yard, companions, feed, tack, handler, rider. Does his tack fit properly? Young horses change shape a lot, he could be in pain from somewhere. How about his teeth? Has he been handled or looked after by anyone else, even for just an afternoon?

This could be a physical problem, a dietary one, a bordeom or emotional thing. Try and pinpoint ANYTHING that has changed, however small or insignificant it may seem to you. Horses view things differently and he could have been freaked out by something he saw or did with you, or something you did with him and now he's on edge. How do you treat him when he's like this? Are you kind and calming or do you tell him off? Maybe he's not spooky and has just learnt that he can get out of doing things by acting up?

You know him best, try and work this out from his point of view.