If he wants to study Mandarin in college, I would suggest going 2+2 with Latin and Mandarin (his GC can explain Mandarin being a late addition at the high school if his GC thinks such an explanation is necessary to the schools he targets). The more exposure he can get before college language classes the better.
I think a lot of students are unpleasantly surprised by the fast pace of foreign language classes in college; if he isn’t a ‘natural’ language learner he will only appreciate the deeper base he can get from two years of Mandarin before starting college.