Share 'Why does my horse do this?'

My horse is an OTTB who has the opposite problem of most ex-racers. He won't go! Yesterday we started out on a ride and we got a little ways down the road. He walked slower and slower until he finally came to a complete stop. He stopped in the middle of a dirt road and just stood there. He didn't seem scared; he was just looking around. I squeezed with my legs, slapped his rump, and used every ...