my rabit is very skinny

Question
he is about 3.5 months old, not fixes he is a boy. he eats lots of hay, and is active if you take him out of the hutch.
in the hutch he moves around, he gets vegetables and treats. for a couple of weeks now he has gotten very thin, he used to be a little fatty, now we can feel his bones, his jaw and his back bone can really be felt. we unusually take him out for a walk in and let him run around in the backyard, but we have not for a couple of weeks due to the bad weather we are having, could the lack of running around be making him skinny?? he does not smell and he is still fluffy, he is about the size of a kitten, and his ears are usually up. is there anything that i can feed him to make him get some muscle mass? or gain weight? like i said he eats mainly hay, and i did that cause when i got him 3 months ago he has a mayor mess in his tummy and butt and the vet told me to feed him lots of hay, i got him from a feed store and i don't know what type of rabbit he is. e-mail me if you can help me, my family and i are concern, we don't want him to die. we let him get a out a bit inside the house, but we have a kitten that plays a bit ruff and we don't like him to play like that with the rabbit, he and the older cat get along just fine. could it be fleas? the kitten has fleas and maybe my older cat and the rabbit got them, they have not been scratching and both had a flea medicine on them, but like i said the kitten still has some, what do you think? thanks!

Answer
Dear Jay,

Hay is a very healthy food for all rabbits, but when a baby is growing he needs commercial pellets to provide enough calories and protein for his little growing body.  You need to use a good quality pellets with no seeds or "treats" in it, and you can read about a complete, healthy diet here:

www.bio.miami.edu/hare/diet.html

Feed him all the pellets he wants while he's growing.  Be sure to be vigilant for any sign of runny stool:

www.bio.miami.edu/hare/poop.html

and be SURE he has plenty of fresh drinking water in a heavy bowl.  Sometimes a very thin rabbit isn't skinny at all, but only dehydrated, and this can be fatal.  Please find a good rabbit vet here to help you, if necessary:

www.rabbit.org/vets

I hope this helps.

Dana