I need advice on training a Tennessee walking horse.?

I work on a small private farm helping retrain three off the track thoroughbreds for their new careers as trail horses. But there is a middle aged, ten maybe fifteen year old Tennessee walking horse who's owner wants him put back into work. He is a very nervous horse, always tense around people. He leads great never pulls,backs, stands ok and is generally pretty good. He was a fun good trail horse but he developed the bad habit bucking. he started the bucking and his owner had this guy who was a total creep and bad horseman ride him and that's what probably made the problem worse. I rode him yesterday to see what/ how he would do. He stopped with gentle/barley any pressure on the reins and walked with the slightest of leg cues. They don't have a ring so i rode in a small pasture and he did great in his comfort zone by the barn but at the top of the pasture he got a bit to speedy for me.i am wondering if i should have him in a shank bit like a curb or a tom thumb, just for a little bit extra whoa because I asked for him to whoa and he kept going and went faster. I tensed and grabbed with my legs to much which sent him faster and i was still pulling back for a whoa and he got confused and tossed his head and got a bit panicky .That part was my fault but i didn't expect him to totally ignore my asking him to whoa, not that that is an excuse i wont do that again. what should i do if he panics again? asking for him to whoa makes things worse but what if i need him to stop.