I would say, that after a long holiday from riding, he is having a hard time getting back into the swing of things. Find out exactly what he is getting fed at the new stable he is at, as the feed may be making him hot as well. You are correct to rule out any physical discomforts.
If I were you, I would start at the very beginning with him, as to not induce any arguments. Work with him on simple things, concentrating on 'calm' through out the whole lesson. Make sure he is calmer after the lesson than when he started. When he accepts each lesson, then I would consider it safe to slowly work up. Don't make each session so horrific for him, that he dreads being handled. Find out what his motivation is, be it food or scratching, or being put left alone for a few moments to reward him often for the simplest things he does good, and go from there.
Thanks for all the advice, but after a lot of trying, thought and weighing things up I've decide to send Acey back to his old home. He is just not settling and he really is very green. Turns out hotness or freshness isn't the issue...he is just bright, luminous, sparkling, neon green!!! It would take months of professional training, experience and lots of money to turn him into what I want, and its not fair on the poor fella-he just wants to chill out in the pasture and go on the occasional hack!!
He is going home this weekend...and I am heartbroken :(