I'm new here so sorry for butting in but I'm British and his accent when is speaking English is that of a native English speaker, personally in my opinion. As others have said it depends on where he is living/travelling accent wise.
But as he was brought up with English as a language (i assume bc his dad is American) and spent since he was 9 living in England and educated here. His accent and way of phrasing is that of a native speaker.