Hi any, opinions will vary but my opinion is yes, it is rude. Since you are going to block him everywhere anyway, how can it hurt to let him know? Otherwise he doesn't know. Did you fall off the face of the earth? Did you decide to ignore him? Did he say something wrong? That seems quite cruel to do to someone I think.
Maybe just text or msg one or two lines - I don't think we should talk to each other for now, I need time away. That's it, that's all you have to say then start blocking.