How to make sure my horse won't spook on trails.?

Me and my friend bought a horse in May of 2010.. so I have almost had him a year now. We broke him and everything (he was easy to brake) she moved to Raleigh, and now Im stuck with him(: He is an 11yr old QH/Paint, and was in a stall his whole life! He never got out and if he did it was in the round pin for no more than 2hrs. So naturally he is spooky, and had to 'be taught' how to be a horse from my horses in the pasture.. (he didnt even know how to drink outa water buckets) but he is now pature boarded and I am now only arena riding him, and its going really well.. his only problem is that he has a problem with slowing down at a lope! Riding in the arena gets kinda boring, so thats why I wanna start taking him on trails we have around the barn, and Im scared he will spook like crazyy, and I dont want him to get hurt! Any ide?!