^ Good point above about the fashion majors being segregated. Pratt goes further by having a separate foundation program for fashion, even though you are in the School of Design along with ComD, ID, and interior. Those latter three majors take the same foundation year with one anothe and with the School of Art (fine arts, animation, film, etc.). But fashion has separate first year courses. Another reason might be that the major is at capacity and/or piority registration is given to major; thus, no one outside of it can take those courses. A lot depends on how the particular school is run.
OP and son might also look into SAIC as they have a LOT of areas of focus: fibers, fashion, designed objects, etc. SAIC is a lot more fluid in its curricula. A studio degree can involve study of one discipline, or several.