My Horse wont stop taking off and bucking me off?

I have a 17.2 7 year old chestnut thoroughbred gelding who recently has started taking off with me then bucking me off. It all started 2 months ago when i was hacking him out on the road and a boy racer came up behind me, revved his engine, which spooked my horse and he took off with me on the main road. Ever since then he has been spooking at everything and seems as though it is running away from it and bucking me off. I have been walking him in hand up and down the road and putting obstacles up in the field to try and re build his confidence but nothing seems to be working also i have gone back to basic training on the lunge and long reining and he shows an improvment then once he is tacked up the same thing happens again. Any suggestions as to what i can do???