Ultimately your best and safest bet is a vet visit- even if he does not like to ride. I have never heard about pineapple juice. I would be concerned with the amount of sugar in it. Let me try to find you some more info. on the hairball issue. I also have an excellent link to a woman who is more than happy to help with rabbit, squirrel, etc. problems. Her link looks like it is for squirrels only but it is not so keep looking and you'll find the rabbit part. Email her also as she is great.
Here is one good link
Here is her link
Edited by jwarder on January 6 2006 at 8:57 AM
If my answer was helpful please click the green accept button My answer should not take the place of a Vet's diagnosis when needed. Positive feedback is most welcome and a bonus-though not required-is greatly appreciated no matter how small.