Lean and fit hounds often tend to seem more "ribby" than some other breeds of dogs, especially since you are used to a fluffy pom . If his spine and hips are not jutting out he's probably pretty good even if you can see the ribs outlined. You are actually starting out ahead of the game so to speak by him not being overweight. Bassets are notorious for becoming overweight too easily. As he gets settled he may fill in a couple pounds without adding extra to his food.

By the way congratulations on your adoption of him, I hadn't been to the general area yet but just noticed this thread topic when I stopped by earlier.