Answer

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So whenever I hear that a 4 mos old dog is lethargic and just doesn't want to do anything and is looking sad i usually think there is something going on. It is good that he is eating and drinking so that makes me less worried but a puppy should always act like a puppy and when they dont thats when you worry. It could be something as mild as a little cold or belly ache that will be gone in a day or two. I think making sure he has enough to eat and drink is great and just watching him for now is a good start but if he gets any worse, stops eating, or is vomiting or having diarrhea i would see your vet. If he isnt better by tomorrow it wouldn't hurt to have him examined. If he is not up to date on his vaccinations i would most definitely have him looked at tomorrow to stay on top of things and make sure its not the beginning of something bad.

So keep an eye on him today make sure he is still eating and drinking and not getting worse and if he does call your vet and if not getting better by tomorrow i would talk to your vet then.

I hope this helps