Hi, fellow addict in recovery here Must be great to have a T who understands it from the inside! The one I was seeing recently for almost a year claims that one of his specialties is addiction, but he was painfully under-informed about recovery methods and even gave me suggestions that were downright dangerous...

Do you discuss recovery stuff with your T? If you do, I think it would be a nice gesture to politely acknowledge his sobriety date that he shared with you with a simple note. I would totally do that at least. He decided to confide in you and tell you, so it does not sound boundary breaching to me, it's pretty much therapy material after all, isn't it? Of course only if you think it does not exacerbates your transference or interferes negatively otherwise.