Sakina: Now that you mention it, it does sound like he has dry skin instead of dandruff. The skin on his face gets dry spots from time to time, so I can imagine his scalp may have the same condition. And as for him actually wanting to change his scalp condition: I know he does. He's very embarassed by it and it tends to make him pretty self conscious. Money is a little tight around his house, which is why he's sort of gotten used to the idea of just using whatever's handy instead of going out and buying his own. He's also tried many anti-dandruff shampoos that have failed for him (such as Head and Shoulders - which I know is pretty harsh). Doing all this with no results might have made him feel a little defeated. So there's another reason why I think you may be right about him just having a dry scalp - dandruff treatments just don't seem to work for him.

When I told him about his possible aversions to SLS and buying shampoos without it, he dismissed the idea by saying that such shampoos would be too expensive. I know that's not true. It's just a matter of taking the time to read the list of ingredients. Not only that, he could always just make his own shampoo, which is even more cheaper and healthier than buying most store brands.

Angel Spun: I actually have noticed some red spots over his scalp, so he may very well have chemical burn. He is not allergic to anything as far as I know, but as I mentioned before, he uses whatever is lying around in the shower, which is usually women's shampoo that his mom uses - and it's full of extra fragrances. In the meantime, I'll mention the vinegar rinses to him and we'll see if that helps.

Thank you both for the responses, they've been very helpful and informative.