Your son declared his main spec as Ret and the raid leader accepted that as his main spec when he let your son into the raid as ret instead of the much more desired/needed spec of holy or prot {as evidenced by his asking your son to play holy}.

What if your son had a feral druid with more /played time on it? Then his "primary spec" is feral druid. would that mean your ret pally son could only roll on feral druid drops in the raid? I'm sure that and feral druids in the raid would be totally understanding of losing an item they want to a ret paladin who can't even use it because "feral druid" is his main spec since it has more days /played. Who cares if he abandoned it a year ago. He spent more time on it, so it must be his main still.

If this was a scheduled guild run that your son signed up for and normally goes to as holy while the raid leader goes as ret, then sure, the raid leader should get dibs on a ret drop. But that's not the way it sounds to me. It sounds to me like a guild PUG which is no different to me than a regular PUG except there should be less fear of ninjas in a guild PUG.