Translation:The gentleman has a million dollars.
It is because 'millon' is a noun, and a noun cannot modify another noun. The 'de' is needed to say a million whats. Omitting the 'de' would be like saying "box oranges" in English instead of "box of oranges." Incidentally, 'mil' can be either a noun or an adjective which is why it is OK to omit the 'de' when saying 'mil dolares.'