I would recommend looking at the (possible) language requirement at the schools your son is interested in attending. Most of the colleges my daughter looked at had some sort of language requirement to fulfill during college (or test out of via AP score and/or placement exam).
If your son wants to do Mandarin instead of Latin, he should feel free to change languages. If he doesn’t want to continue to study any language in college, but wants to go to a college with a language requirement, he may want to suck it up and continue with Latin to have the best chance of testing out of a possible language requirement.