What to do about an upcoming meeting between my step son and his biological father?

I raised my step son since he was 3. His father didn’t want anything to do with him, so my wife and I never informed our son about him. Two weeks ago he reached out and stated he wanted to meet our son. I’m not against it because I believe it is better if his biological father plays a part in his life.My son turned 7 two weeks ago and this will honestly be the first time my son meets him and has recollection about the meeting. I have no idea what I should tell my son before the meeting, or if I should even tell him anything. The reason why his father wasn’t around was because he didn’t want his girlfriend to know he had a kid from a previous relationship.I don’t want to keep him away from him because it seems like his dad has changed and wants an opportunity to be a part of his life, but I don’t know how to explain it to my son.Any tips? via /r/Parenting http://bit.ly/2X9CDGE