Hi and welcome
Please continue and step in with syringe feeding. Dry hay should be over 80% of the daily food intake; he needs mostly dry food and he needs a lot more food than you think! Too much greens (including grass) can cause fermentation in the gut and make him feel sick additionally. It can take a lot longer than you think to get a piggy over a problem.
You have already been given the necessary guides by my previous posters.
The need to breathe comes before the need to drink and only thirdly the need to eat. Antibiotics do not just impact on the bacteria in the respiratory system but also in the gut. that means that serious respiratory problem are a double whammy for the appetite. Your syringe feeding support in that respect can really make all the difference whether you can get him through this or not.
If antibiotics (especially zithromax) are wiping out the appetite, I have found splitting the zithromax dosage in two that are given 12 hours apart helpful. Very fresh poo soup from a healthy companion (as described in our syringe feeding guide) or a course of fibreplex have also helped my own piggies dealing with this problem. Both of mine have managed to hang onto their eyes and are still here with me 1 resp. 2 years later, even if it was touch and go for several weeks and they relied heavily on my syringe feeding support. I hope that that gives you some hope and motivation. We can only do our best, but our best can be more than you would think possible.
Fingers firmly crossed!
PS: Please keep any questions and updates to this thread. You can set your own notification alerts in 'watch' at the start of this thread. Keeping the information on an ongoing case together allows us to support you as efficiently as possible and avoid confusing or doubling advice.