TQ had this exact same thing going on - excessive drooling, lethargic, however she also had this panicked look. Like, I'm talking so much saliva that I brought a towel in the car on the way to the vet because I couldn't sop it up fast enough. In our case, she had ended up licking off some of her topical flea/tick meds and was having the reaction most dogs do to it.
We took her to the vet immediately and she is fine. Someone else's dog I know did this as well, and they waited a few days because they were unsure, but her dog is also fine.
I hope Mitch is okay and it is nothing serious!
Maybe Mitch licked another dog's topical flea medication when he was at the dog park?