From guinea lynx. Vitamin C dose.
Oral Dosage: 50mg/kg to treat scurvy, 25 to 50mg/kg for ill pigs, 25mg/kg for healthy pigs
I used 100mg/kg when I suspected my piggy was starting to come down with a URI. (He had some loss of appetite, was not his usual self, but no sneezing and no watery eye). Maybe I should have gone to the vet straight away, and generally the recommendation is to take them to the vet ASAP as URIs develop quickly, but luckily for me, he was well within a few hours (under close watch), after I handfed him some food and kept him warm.
100mg/kg is what I remembered as safe for sick pigs (at least temporarily), but I can't find the reference. I used ascorbic acid, rather than sodium ascorbate or calcium ascorbate.
Anyway, I would not suggest anyone do what I did. You should take your guinea pig to a vet ASAP if he's showing symptoms of a URI, but giving a vitamin c supplement straight away and keeping him warm, fed until you see the vet helps, IMHO.