I am sorry that you have been waiting for a response. I recently just logged onto the site and noted that your question hasn't yet been answered. I am a licensed veterinarian with over 25 years’ experience and would be happy to work with you if you are still needing assistance.
With the symptoms that you have described, it is hard to tell if he is not moving due to pain or if he is weak due to other illness and that is why he cannot get up. Spinal pain, as seen in disc disease can occur along with loss of function or even partial loss of function to the legs and that may be what he is experiencing. The lethargy and the thickened saliva may be due to his getting dehydrated.
It is imperative that he is evaluated as soon as possible. If you are not able to find a housecall vet, you may need to engage the services of a friend or neighbor and get him into a vet clinic. On a dog of this size, it would be best to use a sheet of wook or a stretcher of some kind to lift him, but obviously that would take a few people.
If you would like me to try to help you locate a house call vet in your area, I would be happy to try, but I would need your zip code.
I hope this is helpful. Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please reply so I can clarify for you.