I mainly just want him to know I am not being lazy but I think it will open a whole can of worms :-(

I been suffering depression. My PhD work has really declined in the past few months. I have a letter from my doctor to give to my supervisor.

I don't know if I should give it too him. It's not the sort of thing you'd tell your employer is it? And what is going to think I expect from it. He can hardly make my PhD easier. I'm just afraid he will expect less of me or give me less tasks to do, or think I'm not up to teaching.

I'm not sure what good will come of telling him?