I showed him this and he is just baffled that someone who is a good writer would spend time on something like this…
You could show him the legendary Anthony Bourdain goes to Narnia story?
I think of fic as where I get to go a different way than the canon did (or at least that canon showed). Not necessarily to fix it, but to explore: how did they get rid of the body? What if so-and-so had an abortion? What if SG-1 didn't close the Stargate in time and the alien invasion succeeded? How would the characters react and the plot consequences change? It also gives me the chance to really investigate some of the characters, to try to understand them better.
It's pretty rare that genre media will show the most catastrophic outcomes, right? But you can do that in fic, and get really into the emotional stakes for the characters and the readers. Or you can go the other way and give absolutely everyone a happy ending, which is also generally not the outcome from a tv show or other media. (Witness Star Wars, in which the heroes of the first trilogy end up having everything fall apart in the sequel trilogy!)
My engagement with fandom isn't purely affirmational: I get to dig into the logical inconsistencies and examine them and explain them or fix them. Why did Aslan wait 100 years to help overthrow the White Witch, and how did the Beavers get a sewing machine? Where did Tumnus get tea and toast during 100 years of winter?
I don't write much anymore, but that's some of what drives me to do so. But also: engaging with the fannish community is SUCH a big part of it for many of us.