It depends a lot on your son and the relationship you two have--I'm not sure that there's one way that is right for everyone and every situation. Some things that I think are important:
Be genuine in your concern and about how you feel.
Don't be accusatory or overly confrontational.
Have some treatment options ready for him to decide on when you talk to him--you don't want him to agree that he needs help but then have no way ready to help him.
Share with your son the ways his addiction is impacting you and the people he loves--again, try not to accuse him or make him feel like he needs to be defensive. Be honest but calm.
Talk to him about the consequences of his addiction. Let him know if you are going to stop helping him financially, etc.
I'm sure not everyone will agree with my advice, and that's cool--like I said, the same things probably won't work for everyone. It might help to talk to a professional about the best way to approach your son. He or she doesn't have to be there when you do, but they can help you prepare yourself, learn about his addiction, and come up with treatment possibilities he might consider.
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