Hi
Please have him vet checked for potential congenital cataracts. We can only guess and cannot tell you what is going on for sure without hands-on access and we cannot exclude a potential infection from inturned eye lashes (entropion).
It is thankfully rare for cataracts to develop in newborns or even before birth but it can happen as early as that; most congenital cataracts tend to happen during the second year of life - congenital means that a piggy is born with the genetic disposition to develop them and that they are not an age related problem (which the majority of cataracts is).
If that is the case, then there is no cure. Sight is thankfully the weakest of the guinea pig senses and other parts of the brain will compensate. Your baby will never know what they haven't got and be able to lead a perfectly normal life; they will orient themselves mainly by smell, touch and hearing. Use their scent marks used bedding or uncleaned houses for any changes to the layout. I've had several piggies with congenital cataracts, even to the point of near full blindness, and they have all been able to lead a perfectly normal, enriched life.
Plese ensure that you have a piece of paper with the date in any picture or video and that you have your sales bill as well with any vet bill when dealing with a pet shop. Further information re. your customer rights in this guide on how to beat the fobbing off culture in US pet stores:
New Guinea Pig Problems: Sexing & Pregnancy; URI, Ringworm & Parasites; Vet Checks & Customer Rights