My teen DS has a very flakey scalp, bless him.
Not sure if it's psoriasis (family history, I have small patches myself but it's drier flakes)
I've bought him BodyShop Ginger shampoo to try.
In the past he;s tried Eucerin shampoo , I bought him the conditioner (leave in)
E45 shampoo made him worse
Phillip Kingsley Itchy Flaky Scalp shampoo made no difference
T-Gel made his face sore 
If I need to, I'll take him to the GPs but I want to try something non prescription first
TIA