Son's questions - Birthmother advice requested

First the background. I have two adopted boys, ages 6 and 3. Both adoptions are semi-open/closed. Not sure of the correct term, but we send pictures and letters on a regular basis with the adoption agency acting as an intermediary.We get a couple of letters from the 3 year old's birthmother but we haven't really heard anything from the 6 year old's since he was born.We've always been open with the boys and my 6 year old is starting to mature enough to ask questions. He knows we send letters and wants us to help him send one. The problem is, his questions right now are along the lines of "why did she give me away?" and "tell her I was crying because I miss her."This isn't exactly the type of letter I'd like to drop on his birthmother but I feel like I should probably let her know that he's been asking about her. The letters have always been high level "He's doing this in school, likes playing this, went here for vacation, etc," so not a lot of deep thought stuff.Any advice is appreciated-Matt