So on Friday, he finally did it! He went in a nice round long and low frame at the trot while we were riding on the road. I'm going to be honest and say that I have no idea if he did it on his own, or if I gave him just the right cues and he was like OH! *lightbulb*.
I remember that I was doing nothing at all with the reins, and squeezing gently with my legs, but that's all.
I'm going to ride him again today, and am hoping he will still do it from just squeezing with my legs.
If not... how do I encourage him to go in that frame again?