For me it sounds that he just got warm, either by sun or temperatures outside or by being active. Does he have a cool, shady place outdoors?
Anyway, touch his ears next time - when they feel hot it's time to get to a cooler place if he cannot chose one by himself.
About putting rabbits somewhere and back - no experience with that, it seems to me that for my rabbits it's very important to know the way to their safe place where their expedition started (I don't freak out when one get's out in the woods again, they know their way back home), if I would put a rabbit into another, seperated area I would put him into a (cool) hidy house there and let him go out and explore, but that's just an idea since my rabbits leave their hutches via ramps, or the catflap in my back door, and return by themself.