He is salivating to the point of dripping, he is stand offish, not playing, and doesn't want to eat, also he is panting heavily, he acts like he's scared. Could it be serious?

Hi blupe123... sometimes that can be due to nausea/pain. Will he drink water? Has he vomited that you know of? Normal bowel movement or not? Could he have gotten into anything... the trash perhaps? Does his stomach feel hard? I'd take him in to see the vet to try and determine a possible cause.

Any time an animal stops eating, it's an emergency. There could be many reasons for this. He could have a virus, he could have a blockage, he could have a rotten tooth, he could have pain, etc. etc. So many possibilities.

The best option is to take him to a vet. There really is no way to tell you what's going on online.

I wish you all the best, and please let us know how he's doing, and what the vet says.

Good Luck.

My immediate thought is something like bloat.

You need to go to the vet. Even if it isn't something like bloat, it could be something serious and you will never know until you see a vet.