He should know simply because he has the right to know his ancestry and medical history. he should have the right to choose to have a relationship with his biological parent, or to choose not to because he already has a great dad.

Honesty is the best policy. It's better coming from you than him finding out any other way

My only question is did the bio dad know the child was his, and chose to stay away? Or did you keep him away.I only ask because I think that the answer to that question could really change the outcome of the way the kid reacts to the situation. You should just be prepared to answer all of the questions coming your way! Good luck with it all.

I would talk to his father not the sperm donor and see what he thinks. If he feels it is okay then yeah do it! If he isn't sure about the idea let him sit on it and decide. At the end of it father is the one who raises the kid. Not the sperm donor.. I personally wouldn't wanna change things. Your kid has a great father who has been the father. I wouldn't sabotage that by telling him that he came from someone else's ball sack. 

My parents adopted me and didn't tell me. I was their child and they died of cancer still being my parents. It was until snooping through memorabilia that I found out and part of me wishes I never knew. My bio sperm and egg donor are not good people and it just confirms that my mom and dad did me a favor. I think upsetting someone's world for no read reason is bull. They'll figure out their medical history as they go. I learned nothing I couldn't have lived without. Keep it to yourselves and do them a favor. 

My mom didn't tell me my dad wasn't biologically my father until I was 15. I was pissed she waited so long to tell me, especially when I'd been asking questions for years b/c I felt something was off.

He should have been told long ago. My sister had a baby (my beautiful niece) when she was 19 yrs old. She doesn't know who the "sperm doner" is (that's what we call him) my sister broke it off with him before she found out she was preg because he was not a good person. My niece is now 14 but about two years ago my sister told her that (her now husband) was not her real dad. They got married when my niece was 5. She always called him daddy. She started to question things cause she was growing up and obviously knew something was different. Like he wasn't around when she was little. Before 5 yrs I mean. So my sister told her and had always planned to tell her was just waiting for the right moment. Told her when she started questioning things. It went well and she has no intentions of finding her real father if that what you want to call him. This should never be kept from a child ever. Otherwise you lied to him your whole life. He will find out eventually and it would be best if it came from you. If it comes from someone else it will definitely be a lot worse for you and him. 

Wouldn't you wanna know ??

This happened to my best friend when she was 32 and she said that she'd rather not to know. 

You should have told him before he was 19. I have a 9 year old that my husband is trying to adopt. When we had our first sex talk about sperm and eggs, I explained that he had a bio father that supplied the sperm that made him, but that his father is the one that has always taken care of him. I think it's cruel to keep something that big and important from your son. My aunt did this to her oldest, and I'm honestly afraid for his mental health once they do tell him. He's 18.