Even though he is a lead character he rarely ever talks! He is actually trying his best not to exist very hard. Not suicidal, just kinda tired, very tired. His voice should be medium-low and he is constantly tired, very very tired.
This is also the casting for child Kevin because I doubt they sound very different and young Kevin only shows up in a couple of panels, but please use a slighty lighter pitched voice!

EDIT: When I say tired, that doesn't mean he has a lack of sleep, more like he has the lack of will to do anything other than lay on the floor for hours at a time, in one panel he says he is trying not to exist very hard, which is why he doesn't show up as frequently as other characters.