I'm brainstorming for a story and would like your assistance in fleshing out the beginning. I'm not too interested in how the story ends, only the beginning. The story is about a man that discovered he has the power to change his size at will. The catch, he's in the middle of a zombie apocalypse and seemed to have missed the evacuation. Making him the sole, non-zombie character. What are things that he might start doing?
Since he was not able to receive news of a zombie apocalypse, we can assume that he lives alone, have no close social relationships and does activities that do not require much direct access to the outside world. We may infer that they're quite antisocial or introverted in this case, and would most likely experiment on his ability to change sizes in his own room for a couple of days. Because of this he will have very good control over his powers prior to him even coming out.

At some point, he'll be curious as to what the reaction of other people would be if they found out that someone can change sizes. However, upon realizing that there aren't any people anymore, and that he's he only non-zombie person left, he may use his power to kill zombies by dropping large objects on them. Because of his solitary nature, he might continue staying in the city he was originally in, only going outside in his gigantic form in order to resupply what he needs.

Under the assumption that humanity and its military has continued to thrive, the existence of a giant in his city may have spread around. It would be unknown to the higherups of the military whether he's a new form of zombie or just a regular, gigantic human. However, the man, in his giant form, will undoubtedly notice any scouts such as jeepneys or helicopters spying on him. If he refuses to engage, the military may mark him as docile. If seen intentionally killing zombies, then they may mark him as a potential ally.

What he will do in response to the military isn't clear, as he can easily decline it without much recourse, as he'd be able to disappear in front of them by becoming tiny enough to escape, or big enough to instantly defeat the military.

Other things he could may be to experiment on how his size changes his metabolism; whether staying small might mean he can conserve his resources more easily; whether parts of his body stays the same size when he cuts it off and changes size, etc.

For something more directly related to the fact that he's in a zombie apocalypse, he might experiment on understanding how Zombies react to threats based on size.