If your son is being offered an interview from a local alumnus, he should ACCEPT THE OFFER!
If he wants, he can also still do the on-campus interview with a senior student. However, IMHO, turning down an alumni interview is akin to turning down an offer from the college. See: https://yaledailynews.com/blog/2016/03/29/number-of-alumni-interviews-sets-record/
Also see this thread from a Yale alumni interviewer: http://talk.collegeconfidential.com/yale-university/2046409-yale-interviews.html