Hi Harvey
First thing I would say would be is to stay positive and don’t blame yourself. I presume your son has made the decision to take these substances? If so then this is not your fault. Secondly, the fact he has seeked help and is attending meetings and seeing his GP has to be commended. The first step is his admission he has a problem and is always the hardest. If I was you I would tread lightly and don’t badger him about going to his meetings but just casually ask how they are going.
Although it seems there is some damage to his health with seizures now, he is still only 27. This could be just a set back, he has years left to enjoy life with his family and try to get back on track. I really hope your son pulls through this and has a brighter future.