Daughters 4h horse's mane is a missing a chunk and half is on the opposite side!?

Pull his mane even to about 3" on the right side of his neck. You can use a squirt bottle and brush to get it to lie to the right while you pull it, and just work on it every day. As another person suggested, MTG might help thinner areas or rubbed out spots grow back faster, but the greater benefit is that it will keep the spot from being irritated and leading to more rubbing. Start pulling and thinning now and you won't have any stress about the show in a couple months. Then braid with traditional hunter plaits for the show and voila! you are good to go.

For growing back the bald spot:

Rub MTG on it everyday. This helps stimulate the hair to grow. If he is scratching it, you might want to add some corn oil or flax to his feed to help get rid of dandruff.

For training the mane:

Pick which side the mane is supposed to be on. Comb it all to that side and then put a hood / slinkie on. You'll have to keep it on for a while to keep the hair for bouncing back. Banding might work as well... but it is much easier this way.

for the mane on both sides thing, take 2-3 in. sections and loosely rubber band them on one side (usually the right side) and either put some mouse or hair gel in the sections after they are banded, then as for the section missing, you can either live it out, or get extentions, but those can be expensive, we have a foal at my barn and a few months ago before they weaned him, the day before they went to inspection(for the foal to get registered), he chewed off a large section of his mother's mane, and she didn't have time to get extentions, but out of 9 foals, he won. hope this helps ♥

I agree with Bearly Smokin. You should also make sure that he doesn't have lice. If he does he"ll just keep rubbing it off.