Danduff - really bad, more like cradle cap but DS is 13!

[Insularbabygroup1]

Does anyone have any tips for getting rid of really bad dandruff? DS (13)has huge white flakes more like scaly skin, very itchy.
He had cradle cap when he was a baby and the specialist shampoos for this worked quite well but it has returned with avengeance - we've been to the GP - tried all of the prescription shampoos, tea tree shampoo, caffine shampoo, simple baby shampoos, cradle cap shampoo - nothing has worked.
He has a gluten free diet, which helped a bit initially - and otherwise his skin is good - just the odd teenage spot breakout, but pretty clear.
I've read suggestions of apple cider vinegar or olive oil? Anyone tried this or any other treatments that would help him?

Have you tried ketoconazole shampoo (nizoral)? Sounds like it could be seborrheic dermatitis which might respond to anti fungal shampoo. Obviously can't give an accurate diagnosis on here but might be worth a return trip to GP?

My ds has had some success in massaging tea tree conditioner into his dry hair/scalp about half an hour before his shower then combing it through/out while rinsing it off then washing with a tea tree shampoo and spending longer than usual ensuring it is all rinsed out.

Back to the GP for a referral to a dermatologist if the GP has exhausted options

But I would double check he's tried an anti fungal shampoo

With my DS2(11) the issue turned out to be that he just did not wash his scalp vigorously enough
We tried lots of shampoos and conditioners, including Nizoral, and his scalp cleared after I washed his hair on 2 consecutive occasions - he just needed a good scalp massage!
He has very thick, dense hair and I had not noticed how bad things were on his head until his younger sibs had head lice and I combed them all.

Worth a shot?