Could also be because of an Anglo-Saxon ancestor. Fairly sure you can end up with the graphical culture of a grandparent or maybe a great-grandparent.
However, if he's the viceroy of an Anglo-Saxon area, it's QUITE possible that random events might have flipped his game culture to Anglo-Saxon. Not as common as a county converting to the ruler's culture, but it is possible. Do you have Rajas of India enabled? If not, Event 55010 may fire and change his culture to that of his capital. With Rajas on, it's a decision for 500 prestige, but can't be chosen if their liege has their existing culture.
It's not crystal clear if we are talking graphical culture or regular culture here. What does it say on his character display? Screenshot would be useful.