My kid sister's youngest had his Dad die when he was 15...and he had been very sick for about 5 years before he died.I tried to be a substitute male role model for him and we were pretty close during that time.Maybe I played a slightly positive...slightly helpful...role in his life.I certainly tried to.
His older sisters were old enough to have not been affected,development-wise,by their Dad's health issues as much as their kid bother.
Yes, a good uncle could step in if dad is truly not there. Good bless you for doing that, I’m sure the young man can now look back and appreciate what you did. In our case there was a caring brother but he lived out of the country so couldn’t be there for his nieces.