You've been given some good tips on clipping-- just wanted to add, in case you cared about "trends and traditions" at all (he's your horse and if you are not going to show him it really doesn't matter, but still...)
For stock breeds (Quarter Horses, Paints, Appaloosas, et.) and/or horses showing in some western disciplines where the mane is kept shorter and sometimes "banded" for show, the bridlepath is typically cut no longer than the length of their ear. Lay the ear back gently against the neck, and where the tip is, that's where the bridlepath stops. For really long-eared youngsters, you might want to go shorter than their ear. Exceptions could be reiners and cutters, who are shown with a long mane-- they often do not have a bridlepath cut on them, or only a short one.
For hunters and horses shown in traditional English disciplines, the bridlepath is usually only an inch or two long-- long enough to accomodate the width of the tack they will wear, maybe plus a little. This also looks nicer when the mane is braided for English classes, since the first braid will start up near the ear rather than having a big space before the first braid.
Lighter, more upright-necked breeds such as Arabians, Morgans, Saddlebreds, and etc., often are clipped with a longer bridlepath-- sometimes back to where the "break" is at the arch of the neck. On a youngster of this breed, I would start out shorter-- you can always cut one longer, but it takes awhile for mane hair to grow back in to match the rest of the mane, especially if you want to keep the horse's mane long.
"Foundation" breeders and bloodline groups in many breeds often prefer a more "natural" look and encourage a minimal bridlepath, if one is cut at all.
Just thought I would share-- hope that helps, or is at least interesting, LOL!
Last edited by Eastowest; 05-19-2009 at 04:03 PM.