The event, unless one is a Mid, is virtually "anything goes." Many people dressed to the 9's ... and frying in a "normal" graduation day in May/June in Annapolis . And many in shorts and sport shirts. Wear a tie at your own risk.
It's pretty relaxed and none enforciing any dress code.
It can be especially brutal for the parents who are given reserved seating ... in the sun! Take your sunscreen!
Your relative'll be fine if he feels compelled to wear a sport jacket. He'll be good with or without. Guaranteed.