How do you convince someone to quit doing cocaine when it is destroying their life?

[SB420]

someone please give me some advice.. my boyfriend (who i love very much) is currently on probation and he just turned 18. he can't stop doing cocaine and it is a huge problem. he has been telling me for about 6 months that he is going to quit, but i keep catching him doing it behind my back. his parents keep catching him too... and the last time they caught him (about a month ago), they said that they were going to turn him in to his probation officer if they caught him again. he has already violated the terms of his probation once, and so if he is turned in for doing cocaine, he will most likely go to jail since he is 18 now. i know that the only way he has a chance of giving up blow is if he decides for himself that he really wants to quit...but i don't know how to get him to see that cocaine is RUINING HIS LIFE. even if he only does blow one night in a week, he becomes VERY depressed, sad, hopeless, cynical and irritable for the rest of the week. he gets huge mood swings, and is very disrespectful to his parents. if he keeps doing lines, he is going to lose his girlfriend, he will be kicked out of his house, and he will be turned in to his probation officer which means he is probably gonna do some time in jail. i really love him and i don't want to see this happen to him. i don't know if he is physically unable to quit or if he just doesn't really want to quit...does anyone have advice about what i could say to him to help convince him that he doesn't need cocaine???