You may need to talk to the Dwarven king, Grisha and Sir Patrick a couple time to trigger the speech option. Stealing the deed makes no difference. When I talked to him he asked if I knew anything about the theft, then talking to him again I had the option to ask about the dragon armour.
The belt is in a house in Verdistis, where you may have meet two sisters arguing over an inheritance (one of them mentions a valuable belt in passing).
VERY BIG SPOILER ON DRAGON ARMOR SET