I had a gaited-mix gelding who would either trot or do a standie-style pace under saddle. Pacing came much easier to him; it's my suspicion that the pacing was a corruption of a true gait. I did figure out his buttons well enough to get a trot out of him (the same trot you're describing, too, poetry to watch, but nearly impossible to sit
), but if he got fussy or nervous he would go right back to insistent pacing.
Ground poles are your friend. Since you're starting from scratch, my first instinct would be to put him over ground poles and say "trot" as he goes over and picks up the gait. Get him associating the diagonal gait with the word. I'd start on the lunge or in-hand, and move up to trotting under saddle. When my horse got into a "pacing spell," during which any capacity to trot seemed to vanish, I would cue him to trot as we crossed a pole.
My horse seemed to respond to different kinds of pressure. For trot, I had to sit lighter and give a "sharper" leg cue. Not kicking, but a firm "trot now" kind of squeeze. Do definitely post when he trots under saddle, and count the beats. He may need some help keeping that wierd 2 beat rhythm, and the posting and counting (I had an arsenal of tunes that had "trot beats" that I could hum as well) will help you and him.
To get a pace, I would sit deeper, almost stiffen my seat, and "ease" him into gait with my legs. I didn't ask for a pace often, because he wasn't ever secure enough in trot to be "trusted" to not go into a pacing fit for a couple of weeks, and it was the trot that I wanted (we did some showing, and the pace was just bizzarre to sit).
This can be a tough nut to crack, and at the end of the day you may have to make the decision of "trot or gait" as far as a primary gait. It can be done; look at 5 gaited Saddlebreds, but with my horse the trot was always pretty weak as far as maintaining gait through corners or having slow/medium/fast trot.
Hope that was helpful to you. Good luck!