“True”father is the one who will be raising him. Simply having sex doesn’t make one true father. If the biological father plans to be in the picture then the kid will know who the biological father is. This kid might have two men caring for him or one or none. For all we know bio father might have zero interest in raising a child and might not care for his parental rights whatsoever.

If the bio father is out of the picture eventually the kid will learn that he is raised by the father who might not be his biological. It is not uncommon. There is way more to parenting than having sex and reproducing. How the child is conceived typically isn’t something extremely meaningful compare to how the child is raised.

It’s off topic but statistically and historically men often don’t object raising children that might not be theirs. Many suspect and they often might not be fathers but they feel for these children just the same as they were theirs. They are not “products of infidelity” but are human beings. I am very involved with a community of blended families so I speak from experience

Not saying he must stay with the family but he already knows the child isn’t biologically his. So his decision will be informed one