so two chapters every 10-11 days, roughly 52 chapters in a book. 14 chapters to go. 14x10= 140 or 14x11= 154. say 147 days till the end of the book. assuming writing progress continues at the same rate, book might be finished by mid May next year. Give another 3-6months for editing July at the earliest, September-November more likely.
Methinks you forgot an operation. It was chapter 36 on 12/7 and 38 on 12/17. That's two chapters in ten days, averaging a chapter every five days. If there's 14 to go, that's 70 days, rather than 140. Maybe mid--to-late-March, assuming he takes some holiday time off from writing?
I believe it's been said before that once he gets rolling at the end, things go pretty fast. It might be finished even faster. Or as others have said, this could be a final draft that he's making some minor edits on. If most chapters are fine, he could finish sooner. If it's a first draft, then surely there'll be at least one more round of edits.
Whatever version or pace, it sounds like he's on track, which I take as great news.