Has he tried Nizoral shampoo yet? Or he could add 1/4 tsp of baking soda to the shampoo. Only do that once every 2 weeks at most. Or instead he could rinse the hair with vinegar in water after he washes his hair. Either of those should control oil and treat buildup. The baking soda has a high pH and the vinegar has a low pH, so they work differently. And make sure he is using enough shampoo. I told my boyfriend he needed to use more just because his hair didn't smell clean. It didn't smell bad exactly, but not clean. Using more shampoo fixed it for him, but his skin is not oily at all.
There is no medical condition as such which causes oiliness of the hair. I would suggest your husband to clean your hair frequently with a mild shampoo(you can use the ones for oily hair) and don’t apply any conditioner on the hair. Also he can shampoo twice during each wash and apply shampoo on the scalp without piling the hair on top. A good massage of the scalp is also helpful as it ensures an increased blood supply of the scalp and also takes away the oils of the scalp towards the hair.
It makes sense to avoid the use of conditioner as it also tends to make the scalp oily. when shampooing the hair, switch from warm water to cool water after you've thoroughly rinsed away the shampoo.The cool water rinse helps close the pores of the scalp and slow down the oil production. Please do consult a dermatologist if the symptoms persist.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
I noticed this same odor on my husbands head and pillow. It started a couple years ago when he was 58. He washes his hair everyday. I mentioned he should check in with the doctor and ask what causes this change. I also referred to it as “old man smell” because I’ve smelled this same odor on older men. His doctor told him to use shampoo with tar in it two times a week. Neutragena makes one called T/Gel therapeutic shampoo. It totally does the trick! No more “Old Man Smell”!