It could just be his genetics.
I agree with Amy though... add in some omega 3 fatty acids and see if that improves coat texture/quality.
I'd start with a regular fish oil capsule for people. Poke a hole in it and drizzle over food. Start slow, maybe half a capsule every other day. If he tolerates it (some dogs get loose stools), then increase the amount. It's extremely difficult to overdo the fish oil. A therapeutic dose may be as much as one capsule every day for a couple of weeks, then you can back off to every other day, and then a couple times a week for maintenance.
"Having heard all of this, you may choose to look the other way.....
but you can never say again that you did not know." William Wilberforce 1759-1833