You were right to point out the mistake; you were right to stand your ground and calmly insist that the mistake be corrected.
Some people have a hard time not becoming defensive when someone points out a mistake. That is a shortcoming on their part, not yours, so please don't feel responsible for someone else's emotional state.
Though it may be uncomfortable dealing with this clerk again, if you like the store's merchandise and if the other people working there are nice, then try to continue doing business with them. As long as the prevailing attitude at the shop is of courtesy to customers, don't penalize the owner for the demeanor of the son. If it would make you feel better to speak to the owner about it, then do so... but I'd say, just see what happens if he waits on you again. If there's a continued problem, then by all means speak up to the owner. Otherwise, presuming that you got your money, let it go. Life's too short to spend much time stewing.
I manage a quilt shop, and our policy is that if we make an error, we cheerfully and promptly correct it. But... the customer isn't necessarily [/u]always[u] right, sometimes they make mistakes too! For us, that's when it gets really awkward.
As to problem staff... personally, I watch to see if a problem is a one-time occurrence or if it keeps happening. If I think it is a one-time thing I don't say anything (micromanaging does not help the staff's mood). But if there is a pattern, then I speak to the person, privately.