Hi! Please take a deep breath!
It is good that you have him vet checked; please be aware that every vet will treat for URI first and foremost at least hint, just to be on the safe side. A runny nose can also be indicative of something stuck, so ask your vet to check for that, too.
Please also check for air fresheners and any perfumed products you use from cage bedding to your own body care; they can cause Crash's symptoms, too. Place a bowl of steaming water next to cage to help ease his breathing and keep that refreshed.
That said, Crash is no danger of dying! The really virulent nasties are thankfully fairly rare; if Crash had got one of them, he'd been very ill at the vets several days ago. When searching online please be aware that what you get is not the real reflection; you get all the horror stories but not the many more success stories unless it is some kind of 'miracle cure' which often doesn't stand the test of time. If you have an inclination for anxiety attacks, please don't go there; it will not help your piggy and only make you miserable. You are always welcome to check with us if you are worried and we will help you with whether you need to see a vet and how quickly.
Please get hold of some probiotic from a pet shop. The antibiotic you will be given is most likely baytril, which can in some cases (again, from our own experiences only a minority) affect the gut bacteria as well as the bad bacteria - as can happen with any antibiotic. You give pinch either 1 hour before the antibiotic (US recommendation) or 1-2 hours after you give the antibiotic (UK recommendation). Either way is equally valid. This helps to bolster the gut fauna. Probiotics are not a medication, so they are not prescribed by vets.
Please be aware that if it is a URI, his mates have already been exposed to any bacteria from his sneezing, so put him back. Guinea pigs with a fully operational immunal system that are not stressed (unlike young shop piggies), will be able to fend off an acute outbreak of regular URI, which at the worst is what Crash has got. Companionship is more important as a stress easer than quarantine at this age.
Switch to weighing Crash daily at the same time (like before dinner or before his breakfast) instead of once weighing weekly. Step in with support feeding if he loses over 50g and keep in mind that the difference between a full belly and bladder and an empty one can make as much as 40g and is not something to worry about.
We will help you with tips if that is case. Problems will ease once the course of baytril has finished.
if you haven't got scales, cheap kitchen scales from a supermarket are perfectly adequate for keepking an eye on weight loss or gain from one day/week to the next.
How To Pick Up And Weigh Your Guinea Pig
And again - take a deep breath and calm yourself down. Crash is NOT seriously ill and has got every chance of a perfect recovery as long as you have him vet checked tomorrow and step in with any treatment if that is needed.