... the bad chemicals in his head...

For those of you who may struggle with what depression is, a quick guide.

Illness: We all know what this is; a virus, an upset of some kind in the gut, some malady or other that will bring us down. usually the only good thing out of this is time of work/school.

Serious illness: Something far more sever, could maybe be life threatening depending on the illness, it is nonetheless something that could have an effect on your life.

Feelings: Up, down or in between and as mixed as a bag of melted jelly babies, these chemicals dictate mood and to some extent actions resulting from them.

Now then, mix Illness and feelings together and you have depression. Depending on the severity of it you can usually throw in serious illness too.
And that, dear reader, is depression. 
A feelings illness.