My grandson wants to be a costume designer.

He is 16 y.o. and has won awards for comic costumes he made.

Any advice for him?

Costumes AND props...

Mason Gross (School of the Arts) at Rutgers has a costume design program. Worth visiting!

Does he live in this town?  I happen to be married to someone with an MFA in design from Northwestern, who has been in the film business for a good while.  I would suggest not focusing on costume design until grad school.  Get an undergrad degree in theatre.  I would say she could talk to him, but the business demands 70 hour weeks before the commute, so she isn't available too much right now.

FilmCarp's suggestion makes sense.  My niece got her undergraduate degree in theatre.  During her education she did everything from sets, costumes to performance.  She became a TV talent agent.  

Another relation was also theatre major and now is a costume designer for an international entertainment company.  What the have in common:  An understanding of design and costume construction as well as hands on experience in the theatre.  Good skill at sketching design proposals is another key to success as a costume designer.  (Unfortunately, I cannot give you contact information for the costume designer.) 

What I do know is that for both, their ultimate careers were not exactly what they thought they might be as they went to college but the broader theatre major experience gave them unanticpated options and opportunities.