Advice on opening an adoption

I adopted my son 8 years ago when he was a newborn. Although I had told the agency I was interested in an open adoption, they told me that they had discussed it with the birthmother, who had significant mental illness, and decided on just exchanging letters and pictures, which we have done for the past 8 years. At the time I thought they meant that DS's birthmother had asked for the adoption to be closed, and I respected that choice. Later on, I came to believe that the closed adoption was most likely the agency's choice -- I started to think about exploring the possibility of opening it, but didn't actually do anything about it. A few months ago I got a phone call that the birthmother had died. I asked about sending flowers to her mother (with whom I had also exchanged letters and photos) and the agency gave me an address (they called and asked her permission). I just got a letter from grandma saying thank you for the flowers, and begging me to call her and tell her how DS is. She says she's 80 and doesn't know how much longer she'll live, and she wants to know about her only grandchild. She didn't ask to see DS, but only to talk, but I can't help wondering if that's because she's afraid she'll scare me away. I have really bad laryngitis right now, but as soon as it's gone I plan to call and ask if I can take her out to lunch. I would bring a lot of photos, and maybe ask DS to draw her a picture or write her a letter. If that goes well I thought I'd ask if she wants to meet DS himself. My question is, for those of you who have opened an adoption, how did you handle the intial meetings with your child? DS clearly doesn't understand the magnitude of this -- I asked him if he wanted to meet her and got the impression that he didn't really care one way or another. DS can also be kind of easily overwhelmed, especially when people are emotional, and so my thought is that maybe I should set up a meeting at a park or something where he could run off and play if he got overwhelmed? My other thought was that maybe I could invite her to a soccer game or something so that she could visit with him briefly, then watch him play, then visit for a few more minutes? What have other people's experiences with opening an adoption been? Also, given that she may not live long enough for DS to ask the questions I know he'll want answered, what should I be asking, and how should I be documenting? Thanks!