i really would have him checked over even if you've not seen the laboured breathing again, it's just as you said - they do hide illness very well - and if his behaviour is still unusual (being skittish etc), there may be something wrong. also, maybe he was injured in a fight?
if you take him to the vet - and a consultation is not that expensive, i think mine are about £15 - and there's nothing wrong then at least you have the piece of mind. if there is something wrong, you can start getting him better as soon as possible.
you could freeze some tinned vegetables (something like beans), wrap them in fleece or a towel if they're not chewers - just make sure they can't get to the actually tin because they'll get frostbite - they can cool themselves on that. do they have a cooling stone? you could put a few tiles in the freezer and stick them in the cage - i got some absolutely fab granite slabs for free from a tiling company near my work, they'd just taken them off display and were going to bin them.
as for them fighting, i don't really have any experience with this - apart from when we were introducing two chins but we broke the meeting up as soon as there was any sign of hostility and then they decided they loved each other (after two months!!) - maybe it'd be worth starting a new thread on that?
are they definitely both boys?