No underlying health issues, no fleas / skin conditions, just a wiry coat - keen to know what you use that works. He's already on salmon oil which works fantastically, this is just an extra/surface level thought.
On my wire coated schnauzer I use johnsons baby shampoo...or a specific shampoo for wire haired breeds...the key is to rinse very well...on her furnishings I use Pantene spoo and then Pantene conditioner..
do you card the coat? i.e.run a coarse stripping knife horizontally through the the coat to get out the undercoat...it will help keep her follicles free and maybe improve the health of her coat and skin.
I use my horse's shampoo on my pup! Nothing nasty in it and makes his coat come up really shiny. www.showready.co.uk . I also use the detangler/polisher on his coat before a walk as it stops him picking up sticky burrs!