I see these changes in Toto after stressful situations ... i.e. ... we just came back after a trip and both the flights were horribly stressful for all three of us. Toto hasn't eaten in two days and pretty much just lies in her carrier and sleeps! She is still drinking and she will get up and wander around and lie on our laps for awhile. Maximus is probably just under a lot of stress, especially since you recently brought him home. Calling the vet is a good idea just to rule out anything other than all the changes for him. We find too that just cuddling Toto seems to reassure her and she is soon back to normal.
How old is Maximus? You might try giving him a little rice with chicken, cottage cheese, yogurt, etc. ... the only problem with doing that is that I have a hard time getting Toto to eat her dog food after having "the good stuff"!
But we do whatever it takes to get some nourishment in those little tummies! Please keep us updated!!