What is the right time to tell a child about his biological father?

My 8-year-old son was from a past relationship. His father rejected him at birth and I was a single parent.

When the boy was two, I met and got married to my husband now. He has raised the boy as his own since then.

We have two more children. When is the right time to have the talk with him and how do we handle it?

His biological father has never been in touch and I have also never contacted him.

Should we keep quiet until he is 18years? My husband agrees that we need to tell him. The problem is when to do it.