It is unusual to have vitamin C deficiencies if you are feeding a good guinea pig pellet and fresh vegetables. What is his normal diet?
It looks like what he passed was hardened mucus. Are you weighing daily?
It sounds like the vet may have missed something. I take it you read www.guinealynx.info/malocclusion.html ?
- And got the T-shirt
If you don't have time to feed him, then surrender him to someone who does. You took on the responsibility of caring for this pig, and being "too busy" to feed him is the very epitome if irresponsibility.