I agree with a couple of posts here. Do you even need to let him know you are turning it off? He will figure it out of course soon enough but then he will probably be in contact with you himself.
If you let him know now though in the nice way (like Ann put it) then maybe he will not contact you about it because he will already know the reason. Which no contact with you might be better for you at this point?
I think this is a 2 way deal, weather you contact him with the info, or he contacts you wondering why it does not work. Whichever one will be easier on you...
Sending hugs your way.