I believe military protocol goes "salute the uniform and not the man", so if John isn't in uniform, he doesn't get a salute.
It is, however, entirely appropriate (and safe!) for the young officer to brace and call John "sir".
The question I have, though, is whether or not John's tags are visible in that scene, since it would be appropriate for him (as a serving officer) to be wearing them even on vacation, just as the young officer is. If his tags are visible, the young officer will reasonably presume that John, given his age, outranks him.
How would this be handled?
It is, however, entirely appropriate (and safe!) for the young officer to brace and call John "sir".
The question I have, though, is whether or not John's tags are visible in that scene, since it would be appropriate for him (as a serving officer) to be wearing them even on vacation, just as the young officer is. If his tags are visible, the young officer will reasonably presume that John, given his age, outranks him.