What fun to see this question coming up after it was submerged for many years! I too was fascinated with the fact that Folco disappears without a clue, even when Fatty and others reappear at the end of the story.
Here is the very old thread (posted 11 years ago!) for those who'd like to do some forum archeology.
Aiwendil's explanation sums up the reasons from outside the story very well, but for some reason my curiosity led me to follow what
G55 recommended, writing a fan fiction about his fate. It's called
To Everything ... Time and is posted on the Downs' own
fan fiction forum. Alas, lack of time (or rather, too many other irons in the fire) led to its untimely abandonment somewhere in Chapter 6. I keep telling myself I'll finish it someday, since I do have ideas for the further plot and ending, but this is not the day/week/month/year...
Does anyone know of other good Folco fan fiction? I'd love to see what ideas they have!