What has he been taking exactly? Loose stools can be caused by Vitamin C but only if bowel titeration is hit so unless hes done pharmalogical dosage the Vitamin C won't be the cause of this. Increase Electrolytes and use Cayenne to help the loose stools. Again look to drinking proper levels of water and taking himalayan salt!
Doctors come out with all sorts of crap and fake Science against anti-oxidants and especially Vitamin C. Most of them have no idea about Vitamin C and throw around all day hogwash from their indoctrinations. Vitamin C and other anti-oxidants preferably the network must be used during toxic allopathic kill me quick treatments!