I wish I had something to say other than I'm sorry. I can understand how you would reach out to someone you love, especially if concerned about their welfare.
I think maybe a "thinking of you" type card wishing him well and letting him know you want the best for him wouldn't be entirely inappropriate. But drug addiction, the remorse, the self-loathing, isn't something I understand. You'll hopefully find the words that offer support and understanding without making it harder for him to take the steps he needs to make.
In the end, it's not the years in your life that count. It's the life in your years. Abraham Lincoln
It is hard to fail, but it is worse never to have tried to succeed. Theodore Roosevelt
