I seem to recall that you really need something with a button on to anchor the gown at the front (otherwise it doesn't hang properly)?
Good news that your DS has received some help @icanbewhatiwant and indeed that he's going to the graduation ceremony too. A silver lining out of the cloud of not going to the US.
Great news in a way that graduation 'dress' is more varied nowadays, at least for the girls. Presumably most of the young men wear suits still?