Do you have any idea of the general area where your cat found it? Typically, the best thing you can do for them is to put it back in or near the nest... though that one's old enough that it should be weaned already... I would guess 5-7 weeks old? If it's eating grass and drinking water, it shouldn't need milk replacement.
Since it clearly doesn't mind being handled, check it thoroughly for any scratches/bites (ie puncture wounds) from your cat. Those can easily prove fatal even in an adult rabbit (and even with aggressive vet treatment), so if you see any punctures in the skin, I would definitely try to find a wildlife rehab asap that can take it in.