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.
The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.