Is the wet food new or increased? That could fulfil some of his water needs if so.
Ceph can be hard on ome dog's stomachs, but I would expect more water consumption, but maybe there is individual variation?
Switching to a generic thyroid med, may or may not produce the same result. Some dogs don't do as well on it. If the generic is providing less thyroid, it's possible that he had too much thyroid med. What is his weight? Have you checked the thyroid levels after switching?
Sounds like he is a seizure dog - have tick diseases been ruled out?