Giving up custody to grandma

And what is your question?

Taking back son from grandma who adopted him

Was this adoption done through the court system?

You really can't, without years of time and a lot of expensive legal fees.
You need to accept what you did and try to enjoy your bio child with your grandmother.

What country is this, spain, france, US ? If in the US what state.

But if this was a legal adoption though the court, you will have to go back to court and
1. either prove the adoption had legal error and needs to be over turn due to some error.
2. you do a new adoption taking the chlid back.

Why was the adoption done in the first place

If you want help, you need to provide information. You need to explain the circumstances of grandma's adoption, where this adoption occurred. How long ago it occurred. And why you want to overturn it. Also where is the other parent.

We ALSO need to know if your rights were terminated voluntarily or involuntarily.