Hi Mary Jo Cole, thanks for your question.
It sounds like eating should be just around the corner for Mack since he's being so friendly with the two of you. If you have other cats in the household, I would urge you to add a new litter box (we recommend 1 per cat in the household, plus 1--so, 2 cats means 3 boxes), make sure he knows where it is and leave it in an area that is dark and quiet, tucked back a little if you can. If you're not sure if this will work, placing him into a bedroom alone, while it's quiet and dark, with his own food, water and litter box may help. Cats thrive on privacy, as well as a calm, dark environment when they're stressed.
Cat are notoriously good at not only holding their stool but also their urine for sometimes days on end. If stressed, though, they can opt to go elsewhere so keep an eye out for any suspicious puddles or fecal deposits in corners, potted plants, etc.
To encourage drinking, you may wish to mix some juice from a can of tuna or some gravy into some water and offer this instead. Moistening his regular food or offering canned food may also entice him to eat. Some other options for mixing with his food would be bits of cheese, parmesan cheese sprinkled on top, some pure chicken or turkey baby food (avoid those with onion or garlic), shredded chicken or tuna or salmon, bonito flakes (you can buy fish food flakes from the pet section which are super smelly and kitties seem to love), etc. If he's willing to eat a few treats, it may also help to crush a few up and sprinkle them into his food, or add a small amount of yogurt and mix it in (don't go too heavily, milk products can cause gas, cramping and diarrhea for kitties). Don't hesitate to go to Petsmart and pick up 5-10 of the smaller cans of cat food. Whiskas Captain's Catch is the one we love best in rescue and with feral kitties on the street, no one seems to be able to refuse a sample.
We usually become concerned that cats aren't eating by the 3rd day. We expect that he probably ate on Sunday since that's when you acquired him, so those 3 days will start Monday. If by tomorrow evening he hasn't consumed food in a reasonable portion, you'll need to consult a vet Thursday. Cats can start having liver issues after just a few days of not eating, which hopefully Mack will avoid with some of the ideas above for enticing him to eat. More info: https://en.wikipedia.org/wiki/Feline_hepatic_lipidosis
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