I use overheating as a figure of speech. Overheating, over exerting, etc.

As for not passing out that's a sort of secondary function of his "never tired" quirk, specifically that he neither requires sleep nor can he be forced into sleep/unconsciousness. He can still choose to, but usually that's under more pleasant circumstances like sitting in a comfy chair or being really bored during a lecture, not writhing in pain. 

That being said still kinda working out his quirk too. I was thinking that he simply isn't able to sleep, which does help feed his restless and impatient personality since he doesn't like to sit still for long periods of time. Maybe during school he ought to pic up meditation or something. All of it is still WIP.

Anywhoozies. For the sake of compromises I suppose we could stay with bits and pieces. Maybe just shock absorbing boots and something that helps him carry heavy loads. No real armor just yet.