I have so much admiration for someone who can rebuild their life the way you have. One day maybe I will be a perfect person and in a position to judge you- today is not that day.
Everyone has respect for the way the OP recovered from alcoholism. No one is under the illusion that this was easy. I think most people also grasp that the OP's son was better off not living with her when she was an alcoholic, and may even have been better off emotionally not having even occasional direct contact with an addict.
What is far more difficult to understand is why, in the process of her recovery, which was clearly some time ago, the OP did not at the very least get in touch with her son's father and, even if she did not pursue gaining access to see her son because she genuinely thought it wasn't his best interests, at least be enough in touch with his other parent to be aware that he'd died, and left her child essentially an orphan in his teens. Instead she seems to have been building another family, and not allowing her guilt to spur her into any action on her eldest child's behalf.
It's perfectly possible that her son's grandparents are not simply being hostile if he's right and they wouldn't want him to see her -- they may genuinely think that it's likely to affect him badly if done without the kind of mediation/counselling/preparation that is usually offered when a birth parent and a child removed from their care via adoption are reunited.
And I could see their point, if this is what they think. He's lost both parents at a very early age, and now suddenly his lost mother has reappeared, apparently functional, with a whole new family, and no easily-digested reason for not attempting to contact him before this.
I know someone who was adopted from an overseas orphanage 25 years ago, who was recently put back in touch with her birth family -- her father and mother are married, relatively prosperous, and have several other children who are her full siblings. The reason given for her placing in the orphanage (machinations of a senior family member) and why she was never retrieved by her family before being adopted by a foreigner at eighteen months old don't stack up at all, and she's found this understandably very difficult to deal with.
It's a really delicate situation. OP, I think you should immediately pursue specialist counselling for yourself, and encourage your son to do the same. Perhaps an adoption support service might advise, even though the situation is not the same.