Shampoo for flakey (possible psoriasis) scalp

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I need something from a Pharmacy ( though I would go to the GP if need be)

14.6 DS . He has short hair, not greasy.
But a really flakey scalp, doesn't look like dandruff (my mother has psoriasis though it's 99% clear now, so I'm baseing my guess on that)

Tried T-Gel (too drying, made the skin on his face like sandpaper)
Tried SebaMed, doesn't do much.

Any recommendations what I can buy tomorrow?
No allergies known

TIA

Polytar is the only thing that worked for me. I used to get in from the doctors but cant seem to buy it from anywhere dunno where its gone.

Would love something similar.

DP gets dry head and face skin. Not psoriasis but worse than dandruff.

Goatsmilk soap for face. ( we use cowboy comb) and the beer shampoo from lush was pretty effective.

T gel is great for really flaky skin.