So your son was literally in possession of drugs, which he purports to belong to somebody else but which he for some reason agreed to hold for that other person? Then he was in possession of drugs.

You appear to be suggesting that somebody other than your son was charged with possession of the drugs after your son tossed them behind the couch, and that the person facing the charge wants your son to confess that he was the person who possessed the drugs and that he tossed them behind the couch. As if it needs to be said, your son would want to talk to a criminal defense lawyer before confessing to crimes.

You have told us that your son was charged with something... possession of alcohol? And that he was sentenced "to do an online counseling course for alcohol and drug abuse". From what you have told us, as his sentence includes "an online counseling course for alcohol and drug abuse", then he is "covered by the counseling program" that is part of his sentence. If you're asking us to guess as to the result of successful completion of counseling, such as that the charge might be dismissed, we are not in a position to know or to investigate. Presumably you know the terms of your son's sentence. If your son is confused, he can try inquiring with the probation officer in charge of his case.

The prosecutor will charge defendants based upon the evidence. There can be circumstances in which all participants at a party will be charged with possession of drugs or alcohol at the party, or where only some or only one is charged - it depends upon the facts and the prosecutor's policies. Presumably the "boy [can] say that he is being charged with possession and has to pay a fine of $2800" because that's what happened.