Is he just proportionally smaller or actually skinnier? Wick was the runt of his litter, so he was half the size of his brothers and sisters, but still a reasonable weight for his size.

His health may be ill, causing weight gain issues. Also, it’s possible he’s not eating as much food as the other if they share food (hopefully they’re being housed separately at this stage). It can also be before you had him, he was neglected for being smaller, thus did not have access to as much nutrients. Wick was pretty much ignored by his mom and siblings, so his immune system was a bit weaker and he ended up with a URI.

Give him a bit more than you do the other brother, making sure its unlimited alfalfa pellets and alfalfa hay.

Below is a short clip of Wick, who is currently 11mo. He’s still a bit small for his age, but look at his proportions and let me know if your skinny rabbit seems markedly skinnier: http://www.instagram.com/p/BZmz48alQZ7/?taken-by=wick_theboogeyman
— Wick used to be underweight, but now he’s not. He stays pretty lean.

Hi! I advise you to separate them during feeding times to make sure they are both eating the same amount of food. Bunnies eat fast and it is hard to see whether one isn’t eating more than the other.

It can be hard to tell with bunnies sometimes. Panda and Fernando look very plump and fluffy, but their actual bodies under all that fluff seem to be fairly slender and rangy from what I can tell. I do understand that you ought to be able to feel their bones (especially their spine) if you gently palpate them, so that’s not necessarily an indication that they’re too thin, rather if you can’t feel their bones, they need to lose weight. They’re eager and enthusiastic eaters of everything served to them and their eliminatory habits are quite satisfactory.