Hmmm....he's too young to vaccinate. So how is Princess counting him in their numbers to the CDC? If they count him as a vaccinated passenger then the count to the CDC is a lie. Then again, the count may allow for his being underage and not include him at all.
As the lesser of two evils I think I'd be inclined to list him as unwell then resolve it at the port rather than potentially, however remote, jeopardizing the entire cruise as being under the 90% requirement.
This falls under the "Why the Heck Didn't They Think of That?" column for the app.