I’m with @Rivet2000: framing the interview, and doing some practice runs can make a big difference. Some points to include:
*everybody gets anxious in interviews, so be ready for it
- remember to breath! remember that it’s ok to take a deep breath and think about your answer for a minute before answering
- have answers to predictable questions ready (eg, why this school, things you like most/least about your school experiences so far, tell me about your ECs, how did you get interested in Math, do you have any questions for me)
- remember it’s a two-way street- you are interviewing the interviewer as well! what can they tell you as an insider?
I suggest finding an adult that your son already knows & feels comfortable with to do a mock interview, and then an adult that he sort of knows to do another. This is simple logic: pretty much everybody does pretty much anything better with a little practice!