when homelander first wakes up after kimiko's blast, he is startled awake, gasping for breath, and he coughs. something he has probably never done before.
but what i find most interesting is his gasps for air, he most certainly has never experienced a struggle with breathing before, at least not in normal, human circumstances. vought definitely experimented on his breathing capabilities as a kid.
here in his reddit AMA he states he barely needs to breathe when someone asks him if he can breathe in space. he can't, he just... can survive on little oxygen, so i am sure he doesn't waste his time breathing much. he can probably fly for a long time before succumbing to asphyxiation, swim underwater for a long time, and yes visit space.
so, when he wakes up and takes a deep breath, his body is likely not use to breathing by itself, he probably now has to manually think about making sure he isn't holding his breath, from the muscle memory of simply not taking breaths. also, a horrible evil thought: the last time he was deprived of oxygen was in the lab as a child, and maybe for a second he thought he was all the way back there again, waking up from another experiment. it is no surprise his cries are awfully similar to that of a wounded animal or child.
a side-note conclusion is that: if their plan was to depower homelander, they wasted it by killing him within 5 minutes. there is SO much to explore, so much that was missed. i wanted to see more of antony play a depowered homelander if he got the time to fit everything in he wanted to. in that specific moment, had we gotten a longer scene, homelander should have been reacting very differently. firstly, the pain of existing in a human body!!!! one that had just been blasted, that is now bleeding from his head. being genuinely disorientated, struggling with balance since he can no longer fly, being able to just feel the weight of his clothes, the heaviness of his boots. stumbling over, and definitely crying or shaking at the sheer overstimulation. we have seen him get overstimulated before.
so i love the brief moment of him catching his breath, because it's one of the only "oh fuck i'm human" moments we get to see.