My first horse wasn't a Standie, he was a gaited cross, likely part Morgan, but he was a pretty set pacer. I'll see if I can give you any useful advice...
Johnny did the same stuff. He would trot in hand (we did lots of halter shows, he was a beauty of a mover when he did trot) and on the lunge, but under saddle, especially if he got nervous, he would pace like a demon. Over the years he got better and better, to where only being confused or nervous triggered it, but it never totally went away. Something that was really helpful to us was to not ease into the trot. I got to where a trot cue was a fairly stern squeeze and cluck. He sort of bounded up into the trot, bypassing any pacyness.
If I were you, I think I would keep with the hill work. Your horse probably doesn't quite have the muscles to trot that he does to pace, and he'll need to build those up. Maybe slowly wean him onto flatter and flatter inclines as his muscles build up. When he does trot, let him go on a loose rein, hand on the buckle. Let him learn how to balance himself and you at the trot without bracing on the bit. As he's trotting, post. Help him with the rhythm. That helped my boy a lot, even if I hummed, or counted the rhythm aloud as I rode. Helping them keep that solid rhythm can do a lot.
When my horse would "relapse" into a pacing frame, one of my tricks was to toss a pole into the arena and cue him into trot as he was striding over the pole. Usually the pole under him was enough incentive to "bounce up" into the trot.
Remember, when he does trot under saddle, no matter where, rub him on his withers like he just invented chocolate while he's trotting. Making it a positive experience can pay dividends.
Hope that was a bit helpful to you... I know how frustrating of a problem this can be at times.