The proportions is good, i can´t really see any flaws in it. The way you drew the character made him look skinny yet a little muscular, combined with the o2 tank and the expression he has makes it look like he envy the plant and are him self clinging to life, the conscept therefore i think is good as is, as any more colour exept for some kind of faded background would rouin the message.

Awesome drawing, keep up the good creativity. This is one of the more telling ones, they seem to be more of you. ^^