If his eyes or his fontanels look sunken in, take him straight to the ER

Sometimes when they're sick they don't eat as much, which is normal. Just be sure he has at least 6 wet/poop diapers. Any less than that is caause for concern.

I wouldn't be worried unless he isn't having wet diapers...taking him to the doctors really just exposes him to more viruses

As long as he is eating and having wet diapers he's okay! It's a good thing they sleep when theure not feeling well