The ASPCA Animal Poison Control Center was great - I talked with a vet tech who consulted with a vet toxicologist - they said the milk and water was fine but I did not have to continue it. Given that the Comet was diluted and he probably only ingested a little, they said to wait and watch him. If he vomits, they said take food and water away for a hour, then give the water back. If he vomits again, I should call back. If he is OK after that, I can give him little portions of his regular food on his regular schedule (no need for a bland diet). They said he might have some gastrointestinal upset. He hasn't thrown up at all, he did cough a little about a hour after it happened but that is all.
Cost $65 for the call, and they provided the information within a few minutes. Plus I got a direct number to call back if I do have further questions or concerns. Definitely worth it for the peace of mind, esp. on a Sunday night/holiday weekend! Thanks ICit, for pointing me in their direction.
I'm still surprised he would drink water that smells like that, but I don't think he even knew there is water in toilets before tonight (we always keep the toilet lids closed). Now I know to be extra careful when cleaning too. He seems to be doing fine.