Well, he won't be far from two stores that are really unique to New York.
School Products (schoolproducts.com) is only about 3-4 blocks from the hotel. It's owned by Berta Karapetyan, who is President of Karabella Yarns and author of Runway Knits. This is a barn of a store, tucked away in a rather crummy office building and absolutely stuffed with beautiful yarn. It is the go-to shop for many professional designers and knitters. One of their specialties is Italian cashmere on cones (prices are according to weight) at great prices. Plus a great selection of more down-to-earth yarns. The staff can be delightful or downright surly, but they're generally helpful and knowledgeable. This is NOT a place where you will find knitters sitting on comfy couches and drinking coffee. It is strictly about business, and the business is yarn at good prices.
Habu (habutextiles.com) is unique in a different way. It's almost around the corner from School Products. This is a wholesale showroom with a retail shop that welcomes knitters, so they carry only their own brand. Very cool and Japanese in feel. Incredible, innovative yarns including silk combined with stainless steel; yarn with silver fibers; interesting yarns spun with hemp, paper, you name it. They also sell kits with the necessary yarn and patterns; one of these would be a nice gift (hint, hint).
I' m suggesting these two stores because they're convenient to your husband's hotel, and because they are so unusual. New York has dozens of nice "normal" yarn shops with good selections of commercial yarns, pleasant staff and nice ambiance. But Habu and School Products are in a class of their own.
PS: Can't you come along on the trip?