I would not tell him about her parents. I would not want him to get a feeling that while he can call some people as dad and mom but in realty he does not have any. While you may feel good about it, this is not good for the child in anyway. In fact you should engage yourself with him in a way that even you forget about this and start believing that he is your real child.

Why do you have to do this. Its not good for the child neither is it good for you. Once you tell this fact to your child he will always think that you are not his real parent whenever you object and that could bring differences in your life. You will also have some bad feeling somewhere inside. Now you are the one who will be his parent and why does the past matter in your lives.

This will be injustice to the child. On one hand you are trying give a home and family to the child while on the other you are making him/her the fact that you have done some great thing and helped him/her.

Definitely not. Maybe you are right in thinking that your child may come to know of original parents later in like but I am sure with your proper upbringing you can solve that issue at that point of time.
Hope this gives you some relief.

It depends on what have to thought of further in life. Do you ever want your child to know the truth of his birth or would you want to be the only parent lifelong. If you do not want him to know the truth ever it is always justifiable to not tell him anything. Even if somehow he discovers later in life you can tell him you never thought of him being someone elses child once you got him home.

YES I think. But definitely not before he can understand things.

NO