I had a horse that did the same thing, there are about three or four reasons that this could be causing it:
A) Check your horses feed, is it making the horse have so much energy that it makes it feel threatned by different things
B) Horses like routin, my old horse Autumn was so used to haveing her grooming bucket on the right side that the day I put it on the left side she looked completly stunned and tried to move away from me as if the bucket was evil, try to put a routin in your horses day so that it calms down
C) you need to bombproof your horse, I like the parelii games wich helps nervous horses a lot, private message me and I can explain the whole game system for you
D) you need to use a stronger lead rope, my nervous horse, Mnyat, we have to use a rope lead rope on him with knots,
E) a training tip, if you feel him start to spooke or start to balk than just stop, don't do anything, just stop, than after he relaxes move forward if he spookes, stop him, start with this on the ground and than when he's ready for the arena have someone lead him around with you in his back and when you feel him tense up or start to shy tell that person and have them stop keep doing this untill your ready to do it on your own than do the same thing start than stop, start doing that at the walk than you can do it from the trot and you should have your horse secure about the arena by the time your troting
Hope it helps!!
Horses are my life, if theyre wasnt such a thing life would suck!