Given the number and frequency of cases that we see on the Forum of young/baby piggies bought from a certain chain of pet shops that develop a URI (or other illness) within a few weeks of purchase, I think it's a fair bet to say he probably has an infection of some type that needs treating with antibiotics (abx) sooner rather than later. It takes at least 48hours for the abx to start doing it's stuff so he really needs to see a vet today (thursday). Unfortunately what starts as a respiratory infection can spread to other areas of the body/into the bloodstream and cause a much more general malaise with other symptoms hence why abx treatment needs to be given rapidly.
If there is fluid in his lungs then the diuretic fruseamide will help remove it but not all vets are familiar with using this in piggies. To be honest, given his young age and history, I don;t think there is much to be gained by focusing the vet on heart or urinary stone issues as these are unlikely to apply to him because of his age. Bugs causing respiratory infections "can" spread and infect the heart valves to cause longer term heart issues of bacterial endocarditis - but the time to pursue this is if the abx don;t work. I would however also ask the vet to check his ears for a possible ear infection and examine his skin for signs of ringworm or mites (because of past case histories)
The fact he's off his food means you need to consider syringe feeding to keep his guts moving, and monitor his weight and his poo output daily in case you need to get some gut stimulants from the vet.
The syringe feeding guide is here
https://www.theguineapigforum.co.uk/threads/complete-syringe-feeding-guide.115359/
Poorly pigs do tend to hunch up and can present with hopping - (as can piggies with Vit C deficiency). Providing exta Vit C (50mg/day) will be beneficial to help boost his (currently challenged) immune system and stave off mites and ringworm, It is also worth investing in some probiotics to help his guts against the action of the antibiotic. Both of these can be added to his syringe food (mushed up pellets homogenised with a hand blender are fine - no need to spend a fortune on Critical care which a lot of piggies actually don;t like anyway!) A guide to all these things is here
https://www.theguineapigforum.co.uk...vitamin-c-overview-with-product-links.115404/
I'm sorry you are going through this - it;s no consolation but sadly you are ot alone and ifyou will find lots of other threads on the H & I board of other slaves who have had similar expereirces (and thankfully, for the most part the pigs make a complete recovery)
Sending healing vibes
x