he might have felt a constant underlying anxiety that came to a head after you got settled
in Costa Rica. Anything else stressful going on
in his life? Any family changes? Moves?
Mucinex is a fairly benign medication, with serious side effects being pretty uncommon, although anything is possible. If I were to guess, I would say this is anxiety from another source, and the timing is just suspect for the Mucinex.
Here is some info on Pediatric Mucinex.