Thanks to all for the very thoughtful responses. My style in parenting is to almost never tell kids what they have to do, but to ask questions that hopefully help them figure out where they need to go. This thread has given me a lot of great ideas in that direction.
My son is lucky/benefitting from a great high school. He'll be eligible to graduate after his third year of college and then will have choices — including going back to get a masters or a second major. We both are lucky to have the benefit of other people's wisdom and experience. Thank you.