First things first ...
Skull maturation ... at the back of the skull is a small knot of sorts. When that knot flattens out, his skull is done growing.
Rally and obedience ... sounds GREAT! Regarding conformation, he'll be judged against others his same age, and then, if he does well there, he'll be judged against other non-champion males (it's rare to have many males that are older than two that are entered in shows but aren't yet CHs, so, most likely, this will be against a selection of younger males). Lots of people show their not-yet-a-year-old Mastiffs and have fun doing it!
Good luck to you (and him)!