The silent problem.
Every day, your husband washes his hair in water loaded with chlorine, limescale and heavy metals. On the scalp, that water dries out the skin, leaves it tight and itchy, and feeds the flaking that keeps coming back on his shoulders and collars.
You try the anti-dandruff shampoos, the medicated ones, the expensive scalp treatments. It calms down for a few days, then the flakes come back.
Because the problem was never his shampoo. It's your water.