This is a very complicated problem. A useful place to start is to think about it in terms of values: right and wrong, good and bad. If the value of following the law, avoiding dangerous things, and doing what is right is stronger, your youngster likely says, "No." If not, the ubiquitous peer pressure to use illegal drugs likely wins.
If your child is using "drugs" resist the temptation to react to the problem by getting angry and lecturing him. Most importantly, do not threaten him. Say instead, "It'll be a great day when you understand you're important enough and valuable enough to treat yourself better. It'll be a better day when you don't run the risk of hurting yourself with alcohol and drugs. You deserve better."
Further, resist doing anything to protect your child from the legitimate consequences his drug use causes. If he has problems with the police, they are his problems. If he gets into trouble at school, it is his trouble. Protecting him from the natural and logical consequences of his behavior only supports the behavior. Your youngster needs specialized drug treatment services provided by qualified child-experts. Nonetheless, his accepting responsibility for his behavior and actions includes experiencing the full consequences of that behavior.
Too often, parents find out about or strongly suspect their child's drug or alcohol use and either say nothing to him or have "a little talk" and do nothing else. Drug and alcohol problems are too serious to simply be left to chance. Talk with your child anytime you are concerned. If your suspicions persist or recur, at a minimum, discuss your observations and concerns with a specialist who works primarily with young people who have substance abuse problems.