For his attention, something my trainer had me do was "take his head away." If you have good control of his entire body you should be able to dictate where his head is. When warming up or the first time in the arena, I will ask for and expect that they keep their head down lower than you want in the class. The arenas in Tulsa and Ft Worth (I'm assuming that is where the shows still are at) are coliseums so they have solid walls going up. By keeping their head down initially you are making them pay attention to you and kinda blocking what they can see up above their head. After you have them listening, let his head come back up to it's normal location for showing.
Another thing for attention is lots of transitions, and really making them work instead of just going around on the rail. Have him prepared for anything. I would recommend schooling on him at some of you local shows, or even any show you go to up to the Worlds. What I would do is if my horse messes up, or acts up, correct him like you would at home and make them get their attention come back to you. For example - if I asked for a trot off in an equitation pattern and he cantered, I would pull him down hard to a stop, back up, and then ask him to trot off again. Yes, I won't place in that class, but they will remember the next time you are in there that you will still correct them in the show pen. It works, trust me. You can do similar things on the rail as long as you don't have anybody behind you and you don't interfere with the class.
For the arenas, they should have scheduled times you can get in their and school. They will not be convenient times, though, often overnight and before the show starts.
For his height, I can't say for the english stuff at paint/pinto. He is too short for a QH HUS class, but would be fine in an equitation class. But each breed is different.
For grooming, make sure you really, really learn how to braid well or expect to pay for a good braiding job. You will want it there.
Hope that helps some, let me know if you have any more questions.