My brothers hair always smell bad. what could be causing this?

Check the scalp: You might ask your brother how often he washes his hair and find out if his hair products have an odor. If your brother isn't going too long between washings or using hair products that smell bad, my next advice would be to check his scalp. Skin infections can sometimes smell bad. If his scalp has sores, is red, or is very scaly, he should see his doctor. He may have a skin condition of the scalp.