I went through this with my 2 1/2 yr old Boris. I came home one day to find he couldn't give me my usual greeting (jumping up and down running around and doing the kidney beening) along with excessive slobbering. All of a sudden his hind legs collapsed and he couldn't do it. Also contracted namonia (spelling?). Took him to the Vet, took x-rays, they revealed nothing! Took him to a specialist right away, blood tests were negative, urine - negative nothing came up. In the meantime he was losing weight couldn't keep his food down and one final x-ray showed his oesophagus did not contract the Vet nailed down to Myasthenia Gravis (Google.com / myasthenia gravis dogs).
Symptoms were - excessive slobbering, hind legs buckling under, droopy eyes/face, laboured breathing, could not keep anything in stomach including water. Laying, sleeping the whole time.
It was so very, very sad.
Please look this up and see if these are your baby's symptoms. There's hope! I was surprised to find it's a people's disease. The medication makes all the difference in the world. I got Boris back with the exception of lose bowels.
I give him medication 2 or 3 times a day for a life time. The medication costs are nothing compared to having him back.
I wish you the best.