I wouldn’t worry at all about the interview if the coach said to do it! The one my son did was a very enjoyable conversation and judging from how soon after it the next contact from the coach was, I’m pretty sure he heard quickly that it went well.
Tell your son to have things to talk about besides the sport and to have questions!! If the interviewer is a student, like an admissions fellow, and not an athlete, he could ask if they have friends who are on teams and he can talk about activities he’d like to participate in and ways to be involved beyond the sport.