Started a good story but no way to contact him to request he finish the story
There are a number of authors on SOL who don't want to communicate with anyone.
His last post was only 4 months ago. He posted 5 chapters on 8/10/2018 and then two chapter in the next several days. Perhaps, he has writer's block or perhaps he wants to write several chapters before posting them.
Sadly, demands for 'more stories' aren't generally successful. Writing is not an efficient career path. You end up working for much less than minimum wage (considering how long it takes to write and then edit and publish a book).
Authors are driven. Writing is a compulsion, writers write because they have to, whether or not anyone reads their stories. As such, they generally don't need prodding.
When someone stops, either a particular story or writing in general, there's usually a participating event: writer's block, plot conflicts, ill health, disapproving spouses, etc.
But in the end, it boils down to a given story simply no longer challenging the author. Either he has no clue how to resolve it, his life has moved on from what drove him to write, or the story issues no longer impact him personally.
Thus, rather than encouraging him to finish abandoned stories, you'll generally have better luck with "I loved story X, and would love to see more of the same (specifying precisely what you loved about it). Authors, if their still writing, always want to improve, so they focus on what works and what doesn't (hence they often get fed up with their lack of success or change), so encouraging them to concentrate, rather than continuing failed efforts, tends to have better luck.
Still, four months isn't a terribly long hiatus, and it's entirely possible he's simply recharging or even searching for new ideas.