First of all, the kid was not behaving well when replied to Randy's comment. I just want to say that before I light the following fuse.
As a veteran of 39 years of dealing with young people from junior high to college age in my teaching career, I have to say Randy was not exhibiting the best of social behavior either. He went in to pick up a rear remote kit, which has nothing to do with welding as far as I know. Maybe I'm wrong in this, but the kits I know of are bolt on items, no welding needed. Therefore, I conclude the "kid" was not talking to him or about him. He overheard or was eavesdropping on a conversation or comment between the kid and another employee, depending on whose perspective you look at it from.
From the point of view of the kid, Randy interrupted a conversation between two people he never saw before with a semi-smartmouth comment. The kid replied in a totally acceptable manner for someone his age in the vast majority of situations in which they find themselves. He reacted in a normal manner for a sub-30 person who is treated a bit rudely by a stranger. He didn't respond to Randy's rudeness with a smile and a "Sorry, sir, but I didn't think you would hear me. I wasn't referring to you and I shouldn't be talking about people who aren't present." He didn't back down from the rudeness directed his way, but treated it as a bit of sarcasm and replied similarly.
He did not treat Randy in the deferential manner Randy apparently felt was due him since he was a customer. Despite the fact that the original comment was not professional, the kid wasn't talking to Randy, nor about him. Randy was rude, eavesdropping, and interrrupting. The kid responded in a justifiably sarcastic or defensive way based on his social experiences. He just needs to learn to treat customers differently than he treats his friends and acquaintances. Consider that he avoided using the common street language that wouldn't get past the mods on this forum but that you hear between kids all the time in public situations, or on construction crews. He didn't cuss or swear, didn't tell Randy to kiss various parts of his anatomy or to perform biologically impossible acts. He treated Randy with a certain amount of respect, but not as much as he could have.
Randy, take your deals and run. The whole thing would never have happened if you hadn't started it with your comment. The kid didn't think he was being rude, and really meant no insult.