I was refraining from posting here because I wanted to see others' responses before I threw my potentially less-educated one in. I do see your last post that he has turned back into the lovely horse you knew before training (which is awesome!), but I figured I would add this in case it helps you in the future (should he get re-wound for any reason) or helps other people reading.
In my experience... Dressage-trained horses, since often ridden with perpetual contact, learn to hold onto that contact emotionally and depend on it for every single action they take. They learn to depend heavily on structure and instruction. And then when you let them go, they go "What?? What do you mean you're not telling me what to do with my feet every single step of the way? What am I supposed to do?" and they turn into a big ball of uncontrolled nerves. I have seen it myself when (some) high-strung and high-level dressage horses have the reins dropped in the first few minutes of a ride, they go "What??
" and run around with their heads flinging around looking for the contact and the guidance.
In the arena, you could work on relaxation and letting him go on a loose rein (will attach a video below). That might help before he even gets to the trail. If he gets to the trail and starts wigging out, you could ask him to ride just like you would in the arena - a half-pass here, a collected walk there, etc. It might give him the structure he's looking for.
The buddy he had on those trail rides for the last couple days could be exactly what he needed to re-teach him how to unwind.