How do I handle the situation without losing my girlfriend?

I was in a relationship seven years ago and we had a son. However, the mother of my son duped a number of men into believing they were the father of the child.

When I discovered this, I let go of the relationship. My former girlfriend insisted the child was mine and I gave her the necessary support. A year later, she started mistreating and neglecting the boy. She used to leave him alone in the house for days, under the care of neighbours.

So I sought custody of the child through the Police and she was compelled to release him. He is now going to P1. I do not know where his mother is, although she calls me about thrice a year to ask about the child.

My challenge is that I am in a relationship and I do not know how to tell my new girlfriend about the child. Besides, my son’s mother hinted recently that she wants to come back. How do I handle the situation without losing or disappointing my girlfriend? I really love her and I do not want to disappoint her.