Hot, nervous horse afraid of leg

My rescue Paso Fino has a few issues under saddle. His groundwork has improved immensely. His saddle work, not so much.

He stands relaxed for mounting, but the moment you get in the seat he is ready to take off. He whoas for about 2 seconds. Usually I just let him go, as he is difficult to hold and I would rather that energy go somewhere. He will trot very rapidly in circles. He literally does not stop moving the entire ride.

I've been experimenting with riding on a loose rein instead of constantly trying to rate him. This usually results in an extended trot. He has not broken into the canter, although I do rate him if it feels like he might.

He will whoa if I pull pretty hard back. I've been using a one rein stop with slightly better results. He circles to a stop, but again does not want to hold it. The minute you release he wants to move forward. Which means I have to keep repeating the one rein stop.

Forget about adding leg. You can't even touch his sides. My attempts at even adding a turning aid, result in him spinning around in a circle. Now he is fine with dangling stirrups on the lunge and hand pressure on his side's. But with a rider up, no way!

I finally got a good whoa at the end of our ride tonight which I used as a good ending point. Of course, by that time he was covered in sweat and getting tired.

He does listen to voice somewhat, and he rates sometimes. He turns really well based on rein alone. But he has a long way to go.

I'm tempted to have someone bring out a bucket of grain and hand feed him while I sit up there. Something to build trust, as right now he is ready to jump out of his skin. He feels like a keg of dynamite.

I will say he really doesn't do anything bad. His attitude is pretty good. He does get annoyed with the one rein stop, but ears go up as soon as we move forward again.

He does not gait. He does a trot without the suspension.