Birth son has made contact but is struggling. Not sure how to answer his questions

Hello, I am a birth mother who has had off and on correspondence with my Birth son, now age 26, for the last 9 years. I was caught off guard the other evening when he called me, on drugs, asking questions. While I have prepared for this since the day I placed him for adoption at the age of 1, I was not prepared for what he told me. He has been struggling with the feeling of abandonment. I tried my very best to help him understand that this was never my intention or the case and give him a background into how the adoption came to be, I can't seem to stop thinking he didn't get it. This was not how I would have imagined holding this conversation, but we don't always get our dream Scenario. I have reached out to his birth mother and she has informed me that he has been suffering with this feeling and other about his adoption since he found out and that is how the drug and alcohol addiction came to be where it is now. I am not sure how to go forward. I would love to have a relationship with him, placing him for adoption was the best decision I could have ever made for him, no regrets. but now I have been struck with guilt. I have 3 other children and I have made it my life to make sure they always felt wanted and never alone. and here I failed with my 1st child. I know I can't not control some ones actions or feelings, but it wrenches my heart. I am actually afraid to make things worse when he asks questions. he went into another rehab after the phone call ended. but he was crying and asking if I could just comets him and hug him. I am so lost. any help or support would be great. He does have an exceptional Adoptive family.