I have legal guardianship of my step son and now the father wants him back, can he take him from me if he signed papers giving him to me?

Depends on the nature of those papers, the time frame and the location. Also the circumstances in which obtained guardianship.

I have legal guardianship of my step son and now the father wants him back, can he take him from me if he signed papers giving him to me?

Yes,he can ask for termination. A guardianship can be changed at anytime.

Where those papers though and from the court and signed by the judge ?

Also as noted, he can try, this was not an adoption, so requests to change guardianship can be done.