I agree with Glenn. It seems your bun got good care by the vet you finally saw. You can still give the fibreplex though, it’s a very good fibre/probiotic product that helps the gut, but in your case your bun needed more meds + meds to treat his chest infection as well. My Bam had a chest infection last summer.We saw a bunny savvy vet and he got much the same treatment as your bun ended up getting. I gave him antibiotics for 2 weeks. He also got a brochodialator (bricanyl) to help with his airways.
As for poop, it could be lack of food in the GI tract as Glenn says. Massages are good, the fibreplex is good. On suspicion of painful gas, many of us give our buns baby gas drops (simethicone). Infacol is a good brand.
Here’s an article that I’ve found to be very useful when dealing with a stasis bun. In your case the chest infection complicates matters, but that is for the antibiotics to clear up. Antibiotics can lower appetite and cause tummy upset. Fibreplex can help because it restores a beneficial GI microbiota.
http://www.bio.miami.edu/hare/ileus.html