Since the movie has been released, we can absolutely confirm here at Winx-Fairies that Nabu is "alive," or rather awake, in the movie. The proof is undeniable. How is he is awake we are unable to answer. It is also unknown as to what Nabu's role in the movie will be.
What had originally happened in season four what that Nabu had fallen into a deep slumber, as said by Morgana in the fateful episode twenty-four, Il Giorno Della Giustizia/The Day of Justice. English Wikipedia says that he had died, however watching the episode in both English and Italian, Morgana says he is sleeping. Why the scene had played out like funeral, we at Winx-Fairies have no idea.
As for the movie, since the Trix break the balance magic in the Tree of Life in the favour of dark magic, does Nabu also lose his magic? What kind of magic does he have anyways?