Hello Dan. I'm Dr. Deb and will do my best to help you today.
I'm sorry for this concern for your dog; the picture you sent certainly demonstrates the problem he's having, doesn't it...at least as far as the drooling goes!
In general, there could be a number of different reasons why a dog might excessively salivate:
1. An infected tooth or infection in the mouth or a foreign body or ulcers. Sometimes a quick peek in the mouth will reveal the problem but typically sedation is required for a more thorough exam.
2. Another possible explanation is mild underlying nausea even though the patient isn't actively vomiting. Some dogs with IBD or other systemic disease will excessively salivate although the former is often difficult to diagnose without a biopsy and the latter would require bloodwork.
3. Pain although usually there are other symptoms such as lameness or lethargy or disinterest in food.
4. Idiopathic which means we can't find an underlying cause. Many of these patients will stop salivating after a day or so and we're none the wiser as to why they started in the first place.
5. Eating or tasting something noxious such as a plant or chemicals although these patients will typically only salivate for a relatively short period of time.
It might be helpful to take his temperature if you could to see if he's running a fever; normal is between 100 and 102.5. If it's high, then a vet visit might be prudent.
Otherwise, you could give him over the counter Omeprazole (aka Prilosec here in the States) at a dose of 1/2 mg per pound of body weight once a day. Another option would be Ranintidine (Zantac) at a dose of 1 mg per pound of body weight twice a day. These drugs may help with nausea.
Diphenhydramine (aka Benadryl here in the States) can help to dry up secretions although I'm hesitant to suggest its use. Sedation is often a side effect and with him already being a little quiet, you won't know if his condition is worsening or if he's just having a drug reaction.
Unless he's running a fever and/or he worsens, then I might continue to monitor him for today but plan to have him seen tomorrow if he's still drooling.
I hope this helps. Deb