My step son has been with me for over 5 yrs. His mom and dad agreed on joint custody and we have had him ever since. August 08 made 5yrs. She now wants to see him. He only knows me as his mom and does not remember her. What can we do?

Why not let him see her?

As long as you are there, what's the issue? It's not as if he's not going to eventually know that you're not his biological mother anyway, and if you prevent her from seeing him now, he will be angry with you later.

Agree to supervised visitation at first and see where that leads.

I see no grounds for a TPR. And if the court ordered joint custody she has every right to see him.

So make the best of it.