If your guinea pig has scurvy, he can get better. Symptoms of scurvy include lethargy, weakness, hopping instead of walking, stiff joints, appetite loss, weight loss, discharge from the eyes or nose, and a poor coat, among others. You can read more about scurvy here:
Adequate vitamin C does eliminate scurvy. It takes about a week to see results. You would need to give 50 mgs. per day. The kind you put in the water isn't good. It makes the water taste bad, and the guinea pig won't drink enough. In addition, the vitamins sold in pet stores have often deteriorated before they even reach the store shelf. Many guinea pigs will eat the orange-flavored tablets made for children. If you use 100mg. tablets, you'll have to cut them. You can also use the liquid vitamin C made for infants. Just put the appropriate amount on a treat.
An optimal diet will also help. The base of a guinea pig's diet should be timothy hay. Guinea pigs should be given all of the hay they want to eat. You can buy timothy hay in bags in pet stores. Fresh produce is also important. It’s the best way for guinea pigs to get adequate vitamin C, and they should have some every day. I recommend this site for great information on diet, including the best produce to feed and what to avoid:
If your guinea pig doesn't recover when given adequate vitamin C and a good diet, there is probably something other than scurvy that is causing the problem. In that case, it would be best to see a guinea pig/rabbit vet. This link will take you to a directory:
If you have more questions, just let me know by clicking on REPLY. I hope your guinea pig reaches a full recovery.