You sound like how my mum use to be with me and ive realised how bad i was.
I couldnt even dream of asking my mum to bail me out for coke.. it sounds like you have done it before and hes basically taking the piss now. I know its hard and its your son, but you have to think of your self now and your health.
My coke habbit is once every month or 2 3 week.. only when i drink.. but my gf said she will leave me with the kids.. so thats its for me. I admitted it all to my mum asking her for help. And i think thats what he needs to do. That stuff is so addictive.. you get 1.. then another and another then u owe 120 by the end of the night. My advice is.. give him 1 last chance.. have a look a coke adiction and what steps to take..tell him he quits it or thats it… but thats up 2 u.. u do what you feel is the right decision