With a morbid sense of humour I'd tell him tha the has to be a famous musician before he did drugs, you can't do them if you don't become a famous musician. But on a more serious side...
There are loads of famous musicians who do not do drugs. Why on earth would his parents say "we'll support you financially if you're going to pee away all your money on drugs, even though we think what you're doing is dead wrong, but you're our son and we're your doting parents."
Maybe you have to be a little cruel to be kind? Tell him that the chances of him ever becoming famous are slim to nil, there's a million people out there trying the same thing, and what happens if he doesn't become famous? He'll be some nobody who's living in his parents basement, unsuccessful with no septum in his nose and is addicted to coke, or acid, or some other narcotic. He'll become dependant on it, become violent, become a major !@#, not have any friends, not have any money, you get where I'm coming from right? He'll turn to selling drugs, he'll be spotted by the cops one day and he'll start freaking out, maybe while he's got some acid stickers or w/e in his possession. It'll be hot out and he'll start running from the cops, and because he'll be dumb from all the coke/acid/whatever he's been doing, he'll put the stickers down his shirt and they'll absorb onto his skin and when he wakes up in the hospital he'll think he's orange juice, and that in no way is a joke.
AND! Coca-cola is extracted from the same plant as cocaine, and, it is inevitable that small amounts of cocaine made its way into the drink. However, this was in the 1800s, and was in no way intended to make the drink addictive or "better." The drink is named after the two key incredients, the Coca leaves and Kola nuts. And "coke" is just the short form of coca-cola.
Coca leaves contain small amounts of cocaine, and people in the Andean region of South America have a long tradition of chewing them for their effects as a mild stimulant, appetite suppressant, and altitude sickness remedy. To make cocaine powder, a much stronger stimulant, coca leaves undergo elaborate processing that involves washes by kerosene and several chemicals.
Coca-Cola used syrup from the coca leaves that probably introduced trace amounts of the active substance into the drink. But concern about cocaine addiction grew in the early 20th century, and in the United States, the Harrison Act of 1914 banned the use of the drug in non-prescription products.
When technology improved enough to make it possible, Coca-Cola started using "spent" coca leaves, which go through a cocaine extraction process, rather than fresh leaves, for flavor.
It gives a whole new meaning to "I'd like to buy the world a Coke." I prefer Pepsi.