Go with your gut. If he's not right by next week, get that appt on Tues and ask for some blood work. I don't know enough about cats to venture a guess about what's going on, but if it were one of the dogs, I'd be taking them in, if only for the flat stare, tense/tired look. You can tell a lot by the tension around the eyes and mouth in dogs.
Our vets used to sort of chuckle when I'd call about one of the dogs not being 'right'--but we'd take them in, anyway. After finding, at one time or another, the beginnings of leptospirosis, a couple cases of anaplasmosis and two hypothyroidisms, etc etc, the clinic no longer questions it. As they say, if you're that much in tune with your animals and
you see something 'off', then it's time to get them checked. You know your pet better than anyone else. So go with your gut. Sometimes it can be a lifesaver, but even if it just gives you some peace of mind, it's worth the trip.
it's just the aftereffects of the metronidazole, which can be a bit 'mood changing' we've noticed.
that by Tuesday he'll be back to normal!