I am new to this forum, but not to the subject.
My best guess is that he will not be created a Duke - at least not yet. He will be created an Earl, and the announcement will state that he will be created Duke of York when that title becomes extinct. Note that "Duke of York" is the traditional title for the second son of a British Monarch, and it is incredibly unlikely that the current Duke of York will have a son. Now, if when that happens, the Duke of Cambridge is King, and he wants HIS second son to be Duke of York, well, that will be a number of years down the road...
Also, keep in mind that the current Earl of Wessex has a higher claim to precedence (i.e., the son of the Monarch) than does "Prince Harry", the grandson of the Monarch. Prince Edward was 7th in the Line of Succession when he married. Prince Henry (Harry) is currently 6th.
The final point is that Prince Charles has strongly advocated for a Royal Family that is strongly focused on the direct line of succession. He may have been limited - for now - in enforcing that with his brother, Prince Andrew, but in this case he has more control.
It would be unexpected, but don't be totally shocked if this happens.