Hello. I'm sorry about the wait.
With the current signs of drooling, lethargy and dribbling urine, I would want to get a good oral exam and bloodwork run. The oral exam is to look for signs of a tooth root abscess. Often times, dental issues will present with drooling, fever if an abscess and decreased appetite.
The bloodwork is to look for kidney disease. Dogs with kidney disease can develop oral ulcers due to the high urea levels in the body. This can also cause increased urination, lethargy and dehydration.
The treatment will depend on the cause. If it's a dental issue, the infected teeth are diagnosed with x-rays and then surgically removed. If it's kidney disease, we treat with IV fluids, antibiotics and monitoring the kidney values daily. These dogs often take 3-5 days in the hospital to get back to normal, so he may need some intense care at the vet.
I hope this helps, let me know if you have any other questions.