
Jane,
if he is on dried food this could be one reason he is drinking excessively.
I would try adding boiled water to his food let it soak so it swells then add a little cold water to make a kind of gravy .
I only give breakfast occasionally dry for the teeth but thier main meals I soak.
If you are really concerned I would try and get a sample and take it to the vets to be tested.
Roni