Don't attack him, he'll only get defensive. Maybe a d&m is in order instead. Ask him why he takes it and tell him that you are genuinely worried - you don't want anything bad to happen to him. And bad stuff really can happen. Taking ecstacy is basically playing Russian roulette with your life - you don't know what's in it. Also, you dehydrate easier which can lead to hospitalisation. If you mix it with alcohol you're basically dead, etc.
But he probably knows all this stuff already, everyone does drug education and hears stories on the news about people dying and so on. So, understanding why he does it will mean that the real problem can be discovered.
Definitely tell parents. And if they're no good then get him to a councilor or something.
Also, take care of yourself and etc. If something does happen to him, it's not your fault, you're doing your best, which is really great.