Sometimes runts are fine, other times there is already something wrong with it that means it will pass on no matter what you do. I can understand the need though. When you say it is skinny, is the belly still rounded and full or is it sunken and crinkly? If the belly is full, then it is feeding and perhaps there is nothing else you can do for it. If it is not feeding, it will pass quickly. You have the option of attempting to hand feed it with something like kitten formula, but the mortality for handreared baby rabbits is sadly very high even with a perfectly healthy baby.
Good luck.