I don't have much experience with this at all (never seen this reaction to PAM) but it might be a good idea to give him a supervised drink of water... Like, supporting his neck so that his chin is barely touching the water so he can get a drink if he wants, then lifting him back away once he's done. I've noticed very sick snakes often won't drink on their own, and I can't imagine how being well-hydrated could possible make the situation worse, but it might help a bit.
Also if the water dish is large enough for him to swim in, I think it would be good to replace it with something small enough that he can only drink out of it. You don't want him to crawl into the water for whatever reason and suddenly have a seizure.