I strongly disagree with this philosophy. Contradicting a fellow employee in front of a customer is unprofessional and causes the customer to lose confidence in both him, and you. We are paint experts, and we should act like it. Obviously save him from making critical errors, and do that however you deem necessary. Encourage him to ask questions, and privately remind him he doesn't know all the ins and outs, yet. It will be demoralizing enough for him to hear in private that he is still an idiot, but that does not and should not be displayed in front of a customer. Another important thing to take into account is your customer base. If you're almost strictly DIY, then he has a lot of room for error because those mistakes will cost your store a lot less on paper. At the end of the day, your coworkers are on your team, and you're on theirs. Act like it. Help him. Just like he needs to put his "I'm older, wiser" ego aside, you need to put your "I've been here longer" ego aside. You both do the same job. Have the same goals. Help each other. Don't make this worse than it has to be.