Definitely don't tell him when there is marital trouble. I was a pre-teen and my parents were well on their way to divorce when my mom told me that my dad was not my biological dad. After a full divorce my mom and non-biological dad remarried. Between the divorce and finding out he wasn't who I thought he was all those years it was difficult to emotionally reconnect with him. I did however reconnect.
I am 42 now and I often long to get answers about my biological father's side of the family. Mostly I am curious about inherited traits that I see in myself and the kids that are so different than what I see on mom's side of the family. Now in my 40's it would be helpful to know what disease traits I need to watch for. Is cancer or heart disease prevalent on that mystery side of my genetic code?
I am often curious about possible half-siblings. I am an only child as far as I know. I miss not having aunts, uncles, and cousins for my children and having neices and nephews to grow to love. It is tantalizing at times to think that I may have a very full family tree out there somewhere. Instead it is just my parents and the kids as far as my side of the family goes. I long to have more family connections.
Out of loyalty to my dad who raised me though, I keep these curiosities and desires to myself. My mom has cancer and should she be in danger of losing that battle, I have determined that I will ask her to give me what contact information she does know. I doubt that I will act on that contact information until both pass on, if even then. But if I do not get it, I will never have it should I need or want it later. At this point, I do not even know my biological father's name.