Just a quick update! I did end up going on the ride. He was already doing a lot better when we arrived. We started out in the arena, and while we didn't get to do a tonne of work in there, he was already relaxing a lot more than he was yesterday - which surprised me, considering he was both at a new place and with a new horse. Although he did have one of his pasture buddies with him.
When we went on the trail ride, he was so good. He had maybe two nervous moments, one where he spooked at a horse and another where a tree fell down. Other than that, he was incredible. We did end up cantering a couple times and he went fine - wasn't lame, didn't trip, etc.
We're still going to get the dentist out earlier, but I do think that it was for other reasons he was so tense yesterday. My first guess would be that he was alone - he is always a lot more tense when he's riding alone, it was just that I was a lot more worried yesterday because of the other symptoms. I would also say he might be getting a bit arena sour - just a guess, I'll see how he is tomorrow, but he looked a lot happier out on the trail.
I'm going to get some more lessons alone, just to get him used to not having his friends. Thanks for all your advice! I'll still check out what you all advised if he doesn't improve.
((Also it could have been my hands as well - I rode him loose reined most of the time today))