As long as he is eating hay then that is the important part.
Pellets aren’t actually needed in the diet so it doesn’t matter if he isn’t eating them.
They may also be different from that which he is used to.
Piggies with a healthy diet get all the vit c they need through their diet, and do not need vit c supplementation and as such we do not recommend any long term supplementation (there aren’t any long term owners on here who supplement with anything to healthy guinea pigs). Supplementation of any kind should only be used where there is a vet diagnosed need as a short course during a period of illness to boost the immune system.
Excess vit c above and beyond their dietary needs is simply excreted in the urine so at best supplementing is a waste of money. At worst It can cause their bodies get used to abnormally high amounts and then reacts with scurvy symptoms if that level drops. Any piggy on long term supplementation needs to be weaned off very slowly and carefully.