Hi there and thanks for your question.
First of all, some dogs will drool in response to nausea. It is possible that he has gotten into something or eaten something that has made him a little naseous. If it is due to nausea, it may help to give him some pepcid (famotidine). Ideally, you should do this on the advice of your vet, but it is generally a safe medicine so you can read about it here:
Next, some dogs will have excessive drooling as a reaction to some over the counter flea products. If you do use such products you may want to see if it is coincidental with their use.
It is also possible that your dog has eaten something that tastes absolutely horrible such as certain insects or toads from the garden.
Another possibility is that there is a tooth infection present.
I hope this information helps! If you have more questions regarding this problem, just hit reply. Or, if my answer is sufficient, then please hit accept. Thanks!