I have done extensive research into this. (NB, am just an ordinary person, not proper research)
There are actually 2 types of dandruff - oily dandruff (yellowish flakes) and dry dandruff (white flakes). The oily one is caused by a fungus and you need Coal Tar or Nizoral (chemist from behind the counter). For dry dandruff, which is linked to eczema and dry skin, which I have, you need to stay away from these as they are too harsh. They help to start with and then you strip away the cells but leave skin irritated, it dries out, more dandruff.
I have found a few thinks work for me:
Oilatum intense clears it up quickly but, long term, can be a bit harsh.
Eucerin do a shampoo (and their skin care stuff is great) and a lotion, both of which you can buy on Amazon. The shampoo is Dermo Capillaire and the lotion is the same.
I then turned to conditioners and found out tea tree / ginger / mint are good and now use a Philip Kingsley conditioner for dry scalp. Avoid Kerastase - too harsh. Kingsley stuff is on eBay and is brilliant.
Also, wash hair less. I know that seems mad but a day with your own scalp oils helps. I have always washed daily but now am every other day and often chuck in an extra shampoo free day if I am at home at the weekends.
The only thing your GP can really prescribe is Betnovate for the scalp. It gets rid of the eczema but stings (alcohol based) and does nothing for the dandruff. The Eucerin scalp lotion is much better.
If you want to spend very little money, and he doesn't mind grease, then coconut oil or a blend of that with tea tree really work. My hair is long and it is a faff to wash out. But, if you got his hair very short, that would probably help.
Also, diet helps. Oily fish (or fish oil tablets). Nuts and seeds. Olive oil on things. And, if it is the oily dandruff, then sunshine works too, as does swimming in the sea. Both of those are bad for dry dandruff and swimming pools are a nightmare.
Basically, if he has dry skin on his head, treat it for dry skin.
Hope that helps.