Dealing with questions about "birth dad"

My situation was very similar. My oldest son, now 15, was raised by someone other than his bio dad from the time he was born practically because his bio dad was never in his life. He never signed over his right and we never persued it because he never bothered us, we never heard from him and it just wasn't anything we really thought about at the time. I ended up having 2 other boys with my Husband (now ex-husband) who was raising him as his (and still does) and he eventually began asking questions at about age 11-12. I guess over the years he had gathered little bots of info here and there and has suspected he was not his bio dad and one night when I was sitting outside with a close friend of mine from high school and my mom he popped the question. nI was floored and completely caught off guard. I never knew what I was going to say when the time came and I froze. My mom ended up answering the question and kept it very brief and made sure his "daddy" loved him no matter what. I was always as politely honest with my son about his dad as I could be. I remained as neutral as possible when it was possible and tried to answer his questions when they came up. A couple of years later he found is "step sister" from his bio dads new marriage and they began talking. Then he and his bio dad began talking, I was not thrilled to say the least because I didn't want him to get hurt. So I mostly listened to my son when he wanted to talk about his bio dad and supported him but kept my eyes and ears OPEN because I knew he wasn't going to ever develope a real relationship with this child and he would end up hurt. That was the first year of his life that his dad ever wished him a happy birthday and it was in the form of a text message. Well, a month or so later was Easter and his bio dad was coming to the Houston area from the Dallas are to see his mom who lives down here. He told my son he wanted to come by and get him so my son could go meet the family. Although my son was not happy with me at the time, I calmly explained to him that while that man may be his father he was a virtual stranger and I wasn't letting him leave with a stranger. However, I told him he was more than welcome to come to the house and spend as much time with him as he wanted. He informed my son that he wasn't going to have time to stop by and see him because he had to get to his moms, who lives about 10 miles from us. My son was furious with me at first but I delicately explained that if it was so important fro him to come see his son that he would have made the time to stop by for a few minutes to see him. As far as I know his dad never talked to him much after that and they don't have any contact at all now and haven't for a few yrs. Unfortunately my son had to see first hand what a jerk his bio dad was but in a way I'm glad because I did not want to bad mouth him to my son and have him show up like a knight in shining armor someday and my son think I was the horrible one all along for "lying" to him. It's a tough situation to be in. I definitely wouldn't say no to any curiosity the child may have because they will be 100 times more eager to see why it's fo forbidden. My advice would be to bite your tounge and be there to guide them, give advice and pick them up should they get there heart broken. Good luck to you... Sorry my post was so long!