I, too, think that he is testing you. However, I think that the first day he started acting up, it could have been because his buddy wasn't there (that is if Bart always went on the trail with you - aka his confidence booster) or it could have been him just being silly. I don't really think it's him being buddy sour (at least not now) because he acted that way with your dad's horse the second time, maybe barn sour, but I would expect him to act more along the lines of turning back home, not wanting to move, etc. Anyhoo, when he got away with it - you calling your dad to pick both of you up - he learned that he, in fact, did NOT have to work if he scared you enough. (I mean, come on! What a life that would be: eating, sleeping, and more eating!
) I do not think the saddle or the sores had anything to do with it. Yes, the sores probably agitated him, but, you already stated that you have ridden him with sores before and he was fine. If it is a treeless saddle, then I doubt that was the issue. Beside that, you said that you rode him in the arena (I am assuming in that exact saddle) and he was fine.
This is after all pain is ruled out: I agree, you should ride him in the arena first and take excess energy away. Make sure he is listening, etc. Then, you should take him on a trail. When he starts acting up, give him something to think about. Move his feet: circles, bending exercises, really anything that gets his brain working. If you feel more comfortable, then do groundwork. Really make sure that his feet are moving, though! When he calms down, keep going as if nothing happened. Keep your body relaxed, and eventually he will relax as well.
I hoped this helped!
& I am no
expert (at all!!), this is just what I think.