I know how hard this is, but it helps me to remember that I am totally powerless over my son's addiction and that I have a responsibility to myself to stay healthy and not let it make me sick too. Me being sick too won't help anyone.
I think it's a good move on your part to stop paying for the cell. I like to make my messages about not supplying money or funding to be about "me", so that it's not up for discussion nor is it negotiable.
An example might be...
"Just wanted to let you know that I can no longer afford to pay for your cell phone. I hope that telling you now will give you time to make other arrangements before the end of the month."
Send it however you feel most comfortable. If his response is less than respectful, have an exit plan in place. "I have to go now, love you" was one of my favourites.
Good luck.
Hugs