What would you say determines what is considered the primary spec?
In a guild - my raiding guild at least - it's a declared spec and remains your primary spec (with enhanced rolling rights) no matter what role you happen to be playing for a particular encounter.

In a PuG, it's whatever role you're playing (i.e. retribution in this example), unless otherwise stated at the start.

So you and your son are clearly in the right, and the guy in question is a complete ass.