I canter downhill but my mare isn't downhill. I would certainly avoid it while you have a tripping issue, get him as balanced as possible and working as uphill as he can before you try it. As for getting in a habit, just make sure you don't canter up every hill, don't always start at the very bottom and finish at the very top, if you keep it varied he should realize that he's only allowed to do it when you ask.