Sore mouth

Unless he has any heart disease, try chopping fresh pineapple into chunks and freezing it. Sucking frozen pineapple during the infusions reduces both the horrible metallic taste and also the mouth sores. If he cannot have pineapple, sucking on ice cubes is the next best thing.
There isn't a lot that helps once the mouth is sore except tried and trusted topical treatments like Bonjela, Igluu, etc. Ice cream is soothing on a short term basis, as is jelly.