First, take HIM shopping, and pay attention to his wants. If he want the $5K Armani dress shirt, draw the line,
but if he selects the store brand black shirt over the store brand white, get it for him. And if he prefers the white or a grey or ... let him make that decision. He does need a tie. Tell him that if when he gets there, none of the other guys are wearing one, he can take it off, but he needs to start out in one, And once again, let him make the selection. I would suggest when shopping that one of you wear or take something that is a dark purple so you can verify that the clothes selected do not clash with his date's dress.
If his friends are going to the dance, check with their moms as to whether they are wearing sports coats or not; I suppose you can check on ties, too. Or if you are on friendly terms with some of the school staff, call and ask them.

If his friends are going to the dance, check with their moms as to whether they are wearing sports coats or not; I suppose you can check on ties, too. Or if you are on friendly terms with some of the school staff, call and ask them.