Assuming he's young, fit and otherwise well, I'm with the simple GI disturbance opinion. It's known locally here as "a bad pint." Further towards Wigan, as a "bad pie."
Symptoms of which are probably exacerbated by Boyle's Law. Luckily, the effects are generally not long lasting nor chronic.
Ever seen the wrappers on a bread roll expand on ascent. And in the days when one could take a 1 liter plastic bottle of
Roller Cola on an aircraft, how it imploded after that last swig before the seat belt sign went on. Good old Bobby has a lot to answer for, although he does make exceptionally good altimeters.
100% oxygen is helpful for making cabin staff look useful, or, for disgusing the fact that the patient is dead.
Also fatal when the passenger has chronic COPD and the impetus for breathing is hypoxia
