Hi H.! I don't know a whole lot about what would be considered natural, but I can tell you what I do know and you can take whatever you like from it.
I have psoriasis, mostly on my scalp and in/behind my ears, but I do get patches of it elsewhere if I am stressed. My mom also has it, and for her it is much more severe and difficult to control.
The best things we've found for it are salt water and sunshine. We were the healthiest (skinwise) we'd ever been in the summer living in California. Barring that, you can get sea salt baths to soak and Vitamin D from other sources. I make a point to eat a lot of D-rich fish, and while it's difficult to see a correlation between the meals and my skin, I figure it can't hurt. There are also lotions that have vitamin D in them, just aim for an unscented one. For spot treatment when I have a patch of it somewhere new or if it's especially itchy, I use 1% hydrocortizone cream. I don't know if it's the cream or if the patches would go away on their own, but it definitely helps calm the irritation, which is a huge help.
You didn't mention if he has it elsewhere, but I've never talked to anyone with it that didn't have it on their scalp. For that - t-gel by Neutrogena. Maybe a little harsher of a product than you'd like, but I swear, it makes life livable for me. It smells awful, and I don't recommend it for everyday use, but it keeps me sane and my psoriasis under control. (It creeps down my neck if I don't and is utterly icky.)
Anyway, hopefully something I said will help. If you discover something new, I hope you'll share it here, as I'm always on the lookout for something to help.