I think he'd love that one on Candlemaker Row, I hadn't thought of it. Really different to the usual offering, lots of sci-fi/fantasy stuff. And it's very central if you're combining with sight-seeing. Close to Warhammer shop itself and Blackwells so you can shop-crawl.
While we're on it - is there any shop can comes close to Warhammer for customer service. They spend forever giving you advice and talking through products even if you are blatantly not their typical customer. Taught 7yo DS to paint figures. After a fashion anyway.