Charlie has substance — what he needs is shape. Help him practice answering questions in three movements: one clear opening sentence, two or three specific details, and one short closing sentence about what the experience taught him. The closing sentence is where most students lose the interviewer; it is also where Charlie has the most ground to gain.
At the dinner table, ask him a question that requires a story — "tell me about a time you helped a teammate" — and gently coach him to land on a takeaway when he finishes. Ask him: "What did that teach you?" If he doesn't have an answer, that's the work. The habit of finding the takeaway is the single skill that separates a strong interview from a forgettable one.
When practicing, listen for the word "like" and the phrase "kind of." These are not small things. Admissions interviewers are attuned to verbal hedging because it signals a candidate who is unsure of his own perspective. Charlie's perspective is good. The presentation is hiding it.