I really like bosals for when I want to go bitless. It will be hard to find one that will fit such a big horse properly though. To get one that is quality, you may have to get one custom made (big bucks). I don't know if it would be cheaper or easier to find a "loping hackamore" to fit him. They come in all kinds of types from single or double rope to soft and hard braided rope. I would love to get one like this someday ( 4 Plait Soft Rope Hackamore w/Flat Reins | NRS - National Roper Supply - Western Wear, tack, team ropes, horse tack, team roping ropes, bits... ) but until I have that money just laying around with nothing else to buy with it, a rope halter will have to do.
I agree that if you don't think he needs the curb bit that a snaffle is an excellent choice to put him in to start with, there is no real transition issues because the snaffle is not a confusing bit.
I hope you get him and can't wait for some pix.