My horse is crazy when we go out hacking?

my horse is REALLY spooky out of hacks, he jumps about all over the joint, takes of, dips his shoulders, and spins around on the spot. im not really fased by it but it would be nice to have a calm relaxing hack for once, i dont haul of the reins of anything but whats the best way to deal with this behaviour? i take him out all the time with other horses and alone to get him use to it and its been over a year now, he is like 16 years old so he isnt just a young horse playing up. how to you stop a horse doing this? he is awsome in thr arena and he isnt a bad horse he is just genuinly scared of hacking out :L need help please :)