I vote #2. It was ‘impolite’ on the part of the teenage son.

1) What if it wasn’t the teenage son of the person checking out at the grocery store? What if it was a friend/associate of the person at the checkout? Would that change your assessment?
2) What if the teenage son or associate brought a whole basket back to checkout as separate? Would that change your assessment? – we do check line length to see which one to wait in @ checkout (unless you are trying to scope someone out…)

Considering some of the wierdo’s out there, I don’t have too much of a problem with a son/daughter piggy-backing on the parent’s checkout, it may be safer for them depending upon their age. That said, if the parent told them to go to the end of the line, it would have taught the son/daughter some important lessons;
1) Be considerate of those waiting in line.
2) Don’t make last minute decisions (or decisions in general) and expect everyone else to compensate for it.