I don't believe they keep track of those reservation people on any sort of a list. If it were me I would still cancel the reservation for that particular evening.
All dining rooms have the same theme... formal or casual.
That being said, Princess's dress code has been unofficially relaxed recently (since the return from Covid) and you'll find that many of the men no longer wear suits on formal evenings, but that's a decision he'll have to make for himself. My husband just wears his Polo shirt & a pair of his dress pants or even his Dockers.