I would take it to the vet. Sounds like a more Skin related problem then a dandruff issue.
Please don't use human shampoo on you dogs, because dogs have a different Ph balance then we do, and the shampoo will really dry out their skin more so then Doggy shampoo will.
If you have a pet food store in you area, what could also help is a product called "Grizzly Salmon Oil" And what it is is a balance of Omega 3 and Omega 6, that's in an oil form (from fish) and you would take this oil and put in on your dogs food, and as he eats his food with the salmon Oil on it, in a month or so you should see great improvement in his overall skin and coat.
What it should feel like is soft and smooth, no dander, and his coat will be all glossy and shinny from the consumption of the Salmon Oil. At Least in a Terrier I would look for manageable coat (no dander) and at least down by his skin it should feel smooth.
But first contact your Vet and see if it's not a larger issue! :)
Ps: Here's a link to where I work, thee salmon oil is in the Supplements area, Or just search for Grizzly Salmon Oil. They even have them as treats. :)