I hope that your vet appoint goes well!
Animals are like us and they get sick - colds and tummy bugs and things and then they are sick, just like us, they are not themselves.
He is drinking water so that is the main thing - when ever my dogs are sick the vet will always say that ist ok not to eat but when they dont drink thats not good.
Just be with him and keep him comfy and see how he goes.
He may have just eaten somehitng he should not have or just have a bug.
Fingers crossed that he is better soon!
Kate