Story in Analog or Asimov's about a son caring for his mentally declining father who was a science fiction writer

It sounds like "The Fading Pages of a Short Story" by Bud Sparhawk, Analog January/February 2019. The cover can be seen in the last link. I own a copy of the issue, and the story matches your description.

The father is describing an interesting story idea that he has forgotten he already used in a previous published work:

"I just finished the most interesting science fiction story," David said as his son returned to the living room with his coffee. "Got it off my bookshelf." David had been an avid SF reader since—he often reminded Bill—he was a teen. He held up a small magazine whose pages were brown with age.

Bill glanced at the cover. It showed someone in a tight spacesuit staring across a red plain to where a rocket had crashed. "Which story was that?"

"This one," David said pointing to the table of contents. "Written by somebody I never heard of—Walter Stubbins. Damn good story."

Bill nearly dropped his cup. "Dad, that's one of yours."

That's actually the beginning of the story.