I know when I've quit in the past I have what I can only descride as a dead taste in my mouth all the time, and I've felt the need to eat really hot spicy foods all the time. It may just be coming off the fags thats done it, it takes a good while for your tastebuds to come back to life and start tasting things normally again. I also found things smelled a bit strange. This was with and without champix. Give it a bit of time, if he's had all the obvious tests as you say then he shouldn't have any reason to worry about it and hopefully it'll settle down in time. In the meantime tell him to chew gum or something to mask the taste.