I suffer from it too. I also have eczema but ys as you say it doesnt look the same. I get it when I use any shampoo containing sodium laureth sulphate,it gets worse if my hair is greasy, or if I dont dry it with the hair dryer - leaving it to dry naturally makes it worse. It gets so bad sometimes it scabs up, so dont let him scratch it. I have been told I suffer from scalp psoriasis... It gets real bad when Im stressed. Your son could be suffering from scalp psoriasis OR seborrheic dermatitis.
Most often, the scales of psoriasis are thicker and somewhat drier in appearance than are the scales of seborrheic dermatitis (dandruff). In addition, psoriasis usually affects more than one area of the body. If you have scalp psoriasis, you may also have mild psoriasis on your elbows, knees, hands or feet or may notice subtle nail changes. I have dry elbows, hands and feet, also my nails are slighly brittle, especially on my toes.
The best cures Ive founnd are oilatum shampoo, condition well ( conditioner doesnt effect me but check to see ingerients as my eczema isnt that bad now) and dry very well, if you have to use the hair dryer do so on a low heat.
Hope this helps, maybe you could see the GP just to confirm which he could have so that they can give him the best treatment because of his eczema.