If your horse doesn't have a lot of whoa work on that on the ground. Say "Whoa" then give him a second to make the right choice, and then reinforce your whoa with rein pressure. Once they've got it down pat on the ground, move into the saddle. If they're not getting it in the saddle, it may very well be a bitting issue. That hackamore you picked out would be fine if you are a balanced person with gentle hands, if not go for something with a little less shank.