I’m not familiar with the term cecal, just know it has something to do with the internal workings. I havnt gotten to the point of having to flush my bun, but Midnight was quite a sensitive one. If starting fresh, then, Id say start with what is absolutely needed, hay. Bunnys can survive on hay alone, although they may not be the happiest. Theres only two types of hay that my bun can both handle and enjoy, one of which I’m allergic too so she is stuck with one type, timothy, lol. Try 1 hay, give it a week or so. If no poopy butts, then you know your bun is good with that type of hay! If poopy butt is still there after that week, try another hay. Id suggest Timothy, alfalfa (sadly alfalfa caused my little one more poopy butt) and Bermuda. Those were the 3 my vet recommended at least. If still poopy butt, then its possible hes stuck with it
If the poopy butt does subside, then you can start introducing foods and treats, one at a time, id say a week between each new introduction (at least that’s what my vet recommended) and pull away with anything that causes the poops to come back.
I wish you the best of luck with flushing the bun and hope he gets to feeling better!