The OED traces it back to 1978, with a usage in Aviation Week and Space Technology describing a plane as holding xxx pax.

I think the use would predate this?

A bit more digging reveals that PAX is the SITA TELEX abbreviation for Passenger, which is likely how it found its way into aviation jargon. The reason why they chose PAX rather than PAS I don't yet know. I did find out that PAX is both the singular and plural form, and I'm speculating that this might be why. PAS just sounds a bit more singular, as in 1PAS and 2PAS doesn't sound as plural as 2PAX, but I'm stretching.

Probably totally unrelated, but PAS has a different meaning in the list of U.S. Air Force acronyms:

PAS - Political Affairs Strategist
PAX - Air passengers

This list that I found on Wikipedia is a total hoot, BTW, and contains terms like PICNIC, which stands for "Problem in chair, not in computer."