Can you help me with this dum gelding?

I have a 10 year old gelding. I purchased him from a trainer who uses physical punishment for disobedience. I have had him five months and he has turned sour. He reared up for the farrier and reared up on me twice when I hung back from the other horse on the trail. He is gentle when loose in the pasture but a jerk when it's time for me to be in control. I don't want a horse I have to whip on, but it is the only way to make him respond when he is being obstinate. HELP!