I've signed over guardianship to an exboyfriend some years ago n found out my son is getting abused n fell behind in school.. he is not the father of my child n I am the mother.. my child is with me now and doesn't want to go back there.. what legal rights do I have to keep him with me n not return him?

Depends on the terms of what you signed. Guardianships are generally revocable. You may simply have to give him a letter revoking it, or you may need a court order revoking it.

However, until you do, you could be in legal trouble for keeping the child. So you need to move faster and engaging an attorney would be a good idea.

You agreed that the court should award custody to this man. or
you gave the man a paper merely signed by you (no court involved)?
Which?