rabbit repeatedly ill with messy droppings

Question
Dear Dana,
 Our 14 month old male dwarf hotot rabbit started getting ill in December. His poops were either over an inch long or very very small. Some of them had mucous. Some very pudding soft (the big ones)and the small were stone-hard. He was not eating very much and had lost some weight. Our vet prescribed metronidiazole for 10 days, 0.6ml bid; however after five days when his poops started seeming more normal and the antibiotic would make him uncomfortable a couple hours after dosage, our vet told us to stop the flagyl. In February, the same symptoms recurred and we started him back on the antibiotics, this time for ten days. Again, his poops got back to normal and he started eating again. Two weeks ago, the symptoms started back up again, but with less mucous. His poops were very inconsistent, some almost an inch and a half, others were small and hard like stone. He was eating very little. When he hadn't pooped for over 24 hours we took him back into our vet who prescribed metacam for pain and more metronidiazole with the same dosage. She also referred us to another vet who specialized in treating rabbits' teeth. The other vet told us his teeth were perfect. Freddy's weight was 1.52kg, which had been 1.37 in December. The specialist could not tell us what was wrong with Freddy. We are at a loss as to what to do. We are afraid that once he goes off the antibiotics again, he will just get sick again. We stopped the metacam after the first since he did not seem in pain and it was given to him because our vet thought he had teeth pain, which is not the case.
His diet consists of Oxbow timothy hay and pellets for adults as well as 8-10 leaves of kale daily since December. We suspect that it may have been a bad bunch of hay that started it all, as we had a box of very dusty hay that was not Oxbow and may have had mold, we don't know.
 It also seems that after he eats, his stomach gets much bigger right after. When we gently rub his stomach and force him to run around, this is what makes him start pooping again. He really dislikes the metronidiazole and challenges us when we try to feed it to him.
Do you have any ideas or suggestions? Your help is much appreciated. You have been of much help in the past.
Thank you,
Kimberley

Answer
Hi Kimberly

Could you send a photo of a group of his typical poops?  With his coloration, I worry that he might have "megacolon" or what some call "cowpoop syndrome", which is congenital.

I will be better able to tell if I can see a sampling of his poops.  If they are large and misshapen, and somewhat rubbery, it's not good.

Dana