Hi!
Please do not worry about the pellets; they should make only about 5% of the daily food intake at the most. Piggies can live perfectly well without them.
Please weigh your piggy daily; you cannot control the hay intake, which should make 80% or more of the daily food intake otherwise. Do not give fresh food if the poos are slushy, and please never overfeed fruit. The latter is a twice weekly treat if at all. Your piggy is obviously not drinking if it is eating proportionally too much fresh food as it is getting its fluid from there.
Put your piggy on hay while syringe feeding him as much as he needs to hold his weight. A course of fibreplex may help to rebalance the guts. Until then, add plenty of probiotics to his syringe feed. If you have a healthy companion, you can also try to syringe 'poo soup', water in which you have soaked some fresh poos from a healthy companion. Even if they are not the redigested poos, they still contain the gut bacteria to help restock the guts.
Don't re-introduce veg until the poos have been back to normal for 48 hours, and then go back slowly, adding one more veg per meal. Do not worry about vitamin C; your syringe feed formula, the probiotics and the fibreplex are all enriched with it. Hay also contains it.
Complete Syringe Feeding Guide (includes information on fibreplex, which is available online)
Long Term Balanced General And Special Needs Guinea Pig Diet
Complete Syringe Feeding Guide