I was given up for adoption at birth (closed adoption) and am thinking about going though the process of at least getting some kind of medical history about my birth parents. Of course, it costs money, but there is also the option to ask if the birth mother (or surviving family) would be interested in contacting me. I'm half afraid to do this because of the reaction I may get from her or her family. It took a LOT of guts for your son to find and contact you after that much time.
The question is....how do YOU feel about this?
No, it will probably never be a mother/child relationship, although if you signed over custody of him at age 6 he probably does have memories of you that he cherished. But, you weren't there through everything so you can't step into the "mother" role, and I don't think he expects you to.
I obviously can't speak for your son, but I feel there were definite reasons why I was put up for adoption. Do I resent my birth mother for it? Not at all. I figured she did what was best for me, regardless of what the situation was (poor, too young, not mature enough, etc). And, honestly I would like to thank her for her decision that allowed me to be adopted into the greatest family any child could want. I would like to get to know her, and any relatives I may have, but I wouldn't expect anything more from her than a good friend. She could never step into the role of "mom"...that was filled 39 years ago.
I kinda rambled, sorry. I guess what I'm trying to say is, take it slow and don't have high expectations (or pressures on yourself). Just get to know him and his family and don't freak out. Most likely the same things are going through his head.