Once you get him off the high protien feed, he will settle down.
Standarbreds are not as hard to get the weight back on as Thoroughbreds and they settle in very quickly. They are like those "been there, done that" horses. They normally have rock hard, good hooves and legs of iron. They've been fiddled with, hauled everywhere, had just about everything done to them.
They make great remounts once you retrain them. They already know about bridles and harness all over their body. Saddles are not much different. Take it slow with him and he should be fine. Standarbreds gallop all over the field all by themselves, so they do know how to go over a trot

You just have to get them used to a person on their back, and to get used to being allowed to go over the trot.
I rode one in lesson classes when I was younger and he was the favourite among us girls. He was just so laid back and willing to try anything.
Have fun with him!