Bun of that age not growing can be a stomach problem which turns up about a week after their mom weaned them, enteritis has a particular smell and the animals has a dirty butt. Not all of them will feel terrible from it(and die), some just get real skinny for a while before it cures itself.
Best not feed gamebird feed, it's way way way too rich and will give his digestion grief. (check out bunny digestions, it's a weird wonderful mechanism) Buns in general so well on 17% protein, unless it's an adult buck or non-mommy doe, they do better on 14% (or they'll get fat).
As for salads and stuff -- they eat anything safely, and water content is good, they need lots of moisture, the problem with lettuce from the shop is the amount of bacteria on the stuff, not the actual leaves themselves.