My son has dandruff issue. After using several dandruff shampoos like Nizoral, Tegrin, Selsun,etc., the dandruff has not subsided. His hair loss is extreme. Even small hair are falling out. At age of 18 he is more balder than his dad.
Your son should see a dermatologist for his ” extreme” condition. Genetic hair loss can be present in an 18 year old and he should have his scalp mapped out for miniaturization to establish the correct diagnosis. Dandruff usually does not cause hair loss. Without examining him I cannot advise you on why he is losing hair. He may (or may not) have male pattern hair loss, which is independent of dandruff. What you are calling dandruff could possibly be psoriasis. Get a professional to help here, in person.