I was wondering about the extend of the role as well. From the very short description it seems to center on the boy, kinda like in August Rush. So perhaps Jonny's role won't be that large, but that doesn't mean it can't be a good role.
And Jonny is going to be very busy, if Dracula continues for a second season, the TMI movies (I am assuming they will make the third book into a movie as well, City of Glass, I believe that would be) .