These are good suggestions, but I wanted to add that you can usually feel the spine and ribs on a normal weight bunny if you try--they're not like dogs in that they need to have these parts covered by a dense layer of muscle. I too would have him checked for parasites or other problems. Our one bunny that doesn't put on weight as much as his bondmate was recently diagnosed with a tumor (at only age 3), so that is another unfortunate possibility.