6 year old Schnoodle acting strange

My six year old Schnoodle male has suddenly started acting very strange. On Thursday I went to take him out for a walk and he just laid down in front of the door as if he could not walk. I carried him outside and stood him up in the grass only to watch him favor his back right leg and then fall again. He then struggled to get up and just laid there. I took him to the vet later that day and he had a fever of 103...vet tested him for Lyme disease and it came back negative. She moved his back legs in all sorts of ways and he did not show any signs of pain. She gave us anti inflammatorys for the fever and told us to come back if the fever did not go away as she was not sure what was causing it. It is Saturday now and his temp is 101 and he is off of the meds. However he still struggles to get up from the sitting position. He seems to walk differently too. He has no appetite for his food but is more than willing to eat people food. However he is drinking normally. I have been carrying him up and down the steps due to the weakness in his back legs. Wednesday night he was full of energy so the sudden change is a little unnerving. All he wants to do now is lay in his bed and he won’t come when I call him unless I take him out of his bed, to which he follows me, or stands in the doorway.