What should we do with this horse?

Me and a couple of friends go to my neighbors house to ride and work with his horses. When my neighbor rides he gets on and gallops off first thing, without even warming the horses up. My friend got on one of his horses, Brutis, and the horse took off. We think this is because this is what he thinks he's supposed to do. He doesn't stand still at all with a rider on him but he's not really acting nervous. I was wondering if there were any suggestions on ways we could get him to stand and not take off as soon as a rider is on his back.