Hi,
It sounds as though he has a case of dry skin, which can cause this sort of dry dandruff you are describing. Try giving him a daily capsule of evening primrose oil, as this really helps clear this problem up, although will take a few days to show any effect. The other thing you can do is to put some baby skin moisture cream along the affected areas, use bay though, as it is safest if he grooms himself. I use both of these methods on my 9 year old show cat, and they work a treat!
Cheers