My 16 y.o. cat suddenly can't control his rear legs well. any ideas?

I have a 16 year old baby (he will always be my baby) - last Sunday night he went to his litter box and after pooping a little bit he got out and started vomiting saliva (which is fairly common in him) and at the same time he had some diarrhoea but it was like mucus transparent thing.. he later couldn't control his rear legs and they just sprayed like a ballet dancer. We took him to the vet and really thought he was going to die - he barely breathed and was panting. Given the late hour (11:30pm) AND fathers day no one received us at the vet and we came home, thinking he would die in peace with us. My mom and I had the worst night waking up a thousand times to check up on him - he was sleeping between us - and on the morning he was purring but still weak. We took him to the vet first thing in the morning and they checked all his reflexes and told us that maybe because of the diarrhoea he was very weak but I still think that's not it. He walks desperately around everywhere (even if his rear legs sometimes don't work well and he slips or falls). He's barely eating and drinking water though he has peed since the incident. We took him to the vet again today and took some blood tests, including a quick glucose test - his results were normal - 115 and the normal range is between 90 and 120. We are taking urine sample tomorrow and see if the glucose test with the urine matches the blood one.

I'm super worried because I love him with all my life, more than anyone else and I don't want him to suffer but I want to give him the best life possible. Help me please  

Did the vet consider a heart condition or blood clot? Did they take his temperature?

While there are many things that it can be, one thing sticks out in my mind. Panting, heavy, labored breathing, drooling, and loss of rear leg function is a sign of a possible heart condition and blood clot. This is also very painful and potentially fatal.

This sounds serious to me, and I suggest getting a second opinion from another vet.