If it's a bonding issue I agree with Niamh, spend a lot more time off the horse, just grooming, teaching tricks, and grazing, even just sitting by their stall/pasture with them.
If it's not that then you can do a lot of stopings. Set up some poles and cones and make alittle halt course for yourself. Choose poles to stop before and between and choose cones to stop at. Weave through the cones and ask for a random stop.
Make sure your engaging your back muscles when using your half halt, which you should use a constant half halt the entire time you ride until you feel your horse give in and he learns not to bolt.
Though if he is just bolting every so often because he gets scared there isn't much you can do for that, you have to learn to hold on and just go with it, sit up and and enjoy the little ride and then walk him out and let him know it's okay, make him walk past the "scary spot" multiple times.
But if he's really scared of a lot of things stop riding him for a while and instead set up little like "scare" courses. With tarps and balls and scary looking jumps and pool noodles. Make him walk over, through, under and around these scary items, put plastic bags on the end of a stick and gently/slowly rub them on your horse, and shake it by his ears to make noise. It'll help him get used to strange noises/items.
Well first off you can't stop your horse from spooking at something every now and then, but maybe try to some more time with your horse aside from riding him/her. Maybe groom them, wash them, do some bonding exercises etc. So they can build a relationship with you and learn to trust you.