[@Rob]

I mean, where else could I start with power armor? Exoskeletons are literally the bare bones of power armor.

As for fatigue, he kinda does but doesn't suffer from it.. He doesn't get tired out, though he can still sustain injuries and feel pain. So if he works so hard that he strains his muscles he'll know it. His power largely lets him accomplish idle work long into the night.

 As for why he hasn't used this time to get swole, again, he's spent most of his free time (and when you don't have to sleep, it's a a lot more than you think) working on his armor. He also has regular school stuff he's presumably been working on as a kid so he couldn't just go hole himself up in his house to become the perfect body. He's actually quite pudgy.

Edit:

To put it in another perspective, if he had a stamina and a health bar, he has a constantly regenerating stamina bar, but a regular health bar. As long as he doesn't hurt himself he can keep going for as long as he wants, but if he gets injured or his body takes notable damage he has to let his body recover, even if he himself feels fine. 

I suppose his power also gives him a sort of high pain tolerance too since normally when you're fatigued you're tired, but because he doesn't get tired he'll not feel the fatigue.