My new guy did the SAME THING!! He's only 14 hands, though. I let him walk on a loose lead and periodically "test" him by stopping and saying "Whoa". If he chose to keep walking (His MO was to start circling around me), I would disengage his hindquarters and make him circle around me (Not fast, just pushing him a little faster than he was circling himself) a couple of times, then give him the chance to stop again off of the same "Whoa" cue. He stopped every time on his second chance. Now, a few weeks later, he walks calmly at my shoulder, stopping when I stop, often without a verbal cue or even my undivided attention, lol.
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown