How Do You Tell A Coach That A Player On Your Team Is Doing Drugs?

I would tell him straight out. If you think it would be wise, take some one with you as a witness but just be honest

Starship gave you a really good answer.  Whenever possible it is best to be open and up front about this type of issue.  However, I can understand your reluctance and concern that you might be considered a snitch by your teammates.  That is hard.  It could backfire and you lose some friends over this.  You can either request a private meeting with the coach and tell him that you are concerned about this situation but do not want to be exposed to the rest of the team as the one who brought this to his attention.  Any decent coach will understand.  The other thought is to tell you parents and then let them handle it.  Above all else stress that you do not want to be blamed by teammates or connected to an investigation into drug use by your team.  If one member of the team is using then you can bet there are others.

You straight up tell the coach after you pull him aside, and hopefully the coach will tell his parents. It isn't snitching if it could save their life.

If you're worried about others finging out leave a note for him anonymously. Do not hand write it, he may recognize it. Print the note out and slide it under his door or something.