I think he is a both? I tried to looking up what a grand duke is and it kinda seems they have the same hierarchy so I see them as an interchangeable term.
From what I know grand duke is the sibling of the emperor and I think the prince is the one who will be a king in the future
Both. Usually princes (the children of Kings and Queens) have dynastic titles granted by the king or queen. This allows them to own lands and have power, authority, governing rights (otherwise a prince would just be a freeloader with an empty title, particular those who aren't the crown prince). An example is the british royal family. Prince Harry IS a prince because he's the grandson of the Queen, but his actual official title is Duke of Sussex. Prince William's title is Duke of Cambridge.
That's why British people will often correct people when they call William and Harry's wives as Princess Kate or Princess Meaghan. Prince Harry and Prince William are referred to princes because they're of the royal bloodline but their
OFFICIAL titles are DUKES and not prince, so their wives are Duchesses. That'll change if Prince William becomes the crown prince, whose official title is usually Prince of Wales (the reason why Princess Diana was a PRINCESS because, Prince Charles is crown prince and thus Prince of Wales).
Thank you for your explanation
So is he a prince or a grand duke?