As a parent with grown children I can say that most parents don't want their kids overspending for gifts. Personally I would appreciate a 'right size' iPad since it would signify my lessons on money management worked.
OP, sounds like you don't live near him so you may not be able to just look. Call him and start a conversation about iPads and storage. Feel him out and get an idea of what he actually does with it. It may come as a surprise to many kids but we parents usually have lives outside our parental duties. He just may actually appreciate a fully loaded 128 gb with LTE, especially if he travels a lot.
But when all else fails, get the 32 and make it clear that if he wants, you can return it for a different size.