As mentioned, many conditions, infections, medications or combination of these can cause mouth ulcers, sore throat, taste change, and pain.

When my kidney function goes down, I notice taste changes, and in some cases, it can cause mouth ulcers, sore throat, medications like Gabapentin, which I take, as well as pain Meds, can cause taste changes, throat swelling. Antibiotics too can cause taste changes, and conditions like Thrush, a fungal infection, can cause taste changes, ulcers, and pain, but this sometimes needs to be treated with Antifungals, and likewise with a viral infection, like herpes simplex virus, with an antiviral, and mucocitis can cause ulcers to become bacterial infections, and need to be treated with antibiotics. Even dehydration contributes, and has similar symptoms, so it's good he's seeing his doctor who can help, and perform any necessary tests such as blood test, culture test.

Btw, I still carry a water bottle with me everywhere I go.

Good luck