What a lovely letter.
Having an adopted 4-yr, I realized that I hadn't yet thought of my son at age 73. And I must admit that I'm very guilty of thinking that such things wouldn't matter at that point. I'm so very happy for you that you were able to finally scratch that itch.
Also, it seems to indicate that you heritage might be Wiccan, ha ha. I'm writing because I'm awaiting the arrival of what I've been told is my son's full sibling. His birthmother had one, placed one, had another (named for my husband & son) and is now placing the fourth with us. Because what child doesn't want a young-at-heart but actually 55-yr old mom and dad. "It's" due next month on Valentine's Day. It - because like last time she refused to have prenatal. This time however, we were able to persuade her into a doctor appointment at 32 or 33 weeks along. She test positive for a quartet of drugs. Twice. Oxy. Opiates. Benzo-diazipines. Pot. Like a goat in a pharmacy. Not much in her blood, but there. Twice. Could clean up for the 4-days it took to clear her system.
My point - and forgive me for venting, but my point is - I feel as though telling him/them about birth-mom is both cruel and yet necessary if only for their self-esteem.