Hi
Please do not remove any little food Benji is eating on his own; rather try to up his formula feeding amount - you have coming out at the other end what has gone in at the front - and that looks like upping the hay replacement feed is the way forward for you re. poo appearance and firming up.
The impact on the gut from antibiotics is a different problem to dysbiosis (the gut microbiome going wrong through overgrowth of the wrong kind of bacteria) in the disburbance is artificial and will sort itself out on its own eventually after the zithromax has finished. Antibiotic impact on the gut is rather indiscriminate reduction of the biome but it is not an imbalance like in a natural event if that makes sense to you?
Have you enquired at the clinic whether it is advisable to put him on half the dose twice daily instead of once the full dose of zithromax? If not, please do so tomorrow. That has helped with a piggy of mine and made the difference between eating a little on her own/accepting syringe feed and total loss of appetite/fighting the syringe all the time. However, the divided dose will mean that the zithromax is not quite as effective so it needs your treating vet to weigh up the conflicting pros and cons.
You can give poo soup twice daily alongside any fibreplex if that helps.
All the best.