Hi and welcome
With a normal bacterial pneumonia your recovery chances are not bad at all, especially with prompt treatment.
But it vert depends what is causing the pneumonia; there are several bugs and some of them are rather nasty ones if thankfully more rare ones. Only a lab test can you tell you what you are dealing with.
Trimethoprim-Sufla is better known as bactrim (US) or sulfatrim/septrin for guinea pigs. It is now officially licensed for guinea pigs in the UK.
A diuretic is a medication that drains fluid build-ups from the body; it works on the fluid that congests the lungs in a case of pneumonia and helps to ease breathing more quickly. The most UK vet brand names are frusemide or frusol.
The need to breathe comes before the need to drink and thirdly the need to eat. Hence why a serious respiratory infection is always an appetite killer.
Here is our syringe feeding guide, which also contains tips for the amounts and frequency in our comprehensive syringe feeding guide. Please also offer as much water via syringe as he is willing to take.
Emergency, Crisis and Bridging Care until a Vet Appointment
Here is our emergency and crisis care advice collection where we have compiled all the necessary information that you need in a hurry:
Emergency, Crisis and Bridging Care until a Vet Appointment
All the best! I hope that you will notice an improvement within a day or two.