I do the same method of flipping it over on the other side. Works really well.
I keep my horses mane really short, but I'd assume it would work the same on a long mane.
For your horses mane, I would just give it a good wash and condition, brush it out carefully with a paddle brush, put some coconut oil in, and just leave it alone.
Genetically, his mane is on the thinner side, but what you can do now, is keep the hair that's there now, hydrated and strong, so it won't break off, and instead grow all the way down.
Loosely braid it if you want. I'm not usually a fan of braiding, but then again, I have short manes, so I don't have the problem of managing long manes. It does help though,
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