I guess I don't understand what you mean by illegal. He would be considered a mixed breed as far as I know and of course not able to be registered. So I don't know if that answers your question. I have a bullmastiff and I love them. Hope you enjoy your baby as much as I do mine.
A ridgeback should not be on any banned lists. As far as the Bullmastiff part, in some areas, I suppose they are listed. It helps that it is a mixed breed and not a purebred. You would be best to check your local ordinances for this answer, as every place is different.