Bassets are much better off being a little on the thin side because of their tiny legs in the front. There will be less joint problems and less health problems overall if your basset stays trim. If he's energetic and his coat looks good, I wouldn't worry too much. A healthy coat tells a lot about the animal just like healthy hair on humans. If you introduce a new food, make sure to do it gradually, mixing the two together in increasing increments. Otherwise, his tummy will be a mess.