You are doing a great job of being a parent to an adult difficult child who is an addict. One thing that I remember my bro telling me was that the milestones like 90 days sober, or 1 yr sober, were such a big deal that just getting close to them really increased the pressure he felt and the temptation to drink. Some of it was having your parents and everyone at AA know that you were having this birthday and wanting to congratulate you and feeling like he did not deserve the congratulations or birthday party etc.... I have heard other speakers at meetings who also talked about this - both addicts and their family members have spoken about it.
Several months after my parents celebrated bro's 6 mo sobriety he told them he had had a relapse and it was only 4 months and he felt lke a horrible person. It was hard for them to work through - mostly for him. They were just glad he was honest.