Severe but intermittent pain - ideas?

Friends retriever is in a pickle....

History
Had eye removed due to tumour last year - turned out to be benign
Presented with what appeared to be cruciate damage later in the year - turned out to be dyscospondilitis (believed to be from a urine infection that had travelled to his hip/spine)- house arrest for some time and massive antibiotics eventually came right and was back to enjoying life and getting fit again through hydrotherapy / lead walk up hills

Friend went home at lunch time (Friday just gone) and the boy was in terrible pain and could hardly move, After a visit to the vets it’s not looking good. He is on room rest for 2 weeks and then talking MRI scans, looks like neurological/spine... - prescribed Previcox for 2 weeks and room rest. She said she has never seen a dog in so much pain and screamed when he had his head/neck touched, interestingly his blind side.

By home time on friday vet reported he was right as rain as if nothing had happened?

Vet who observed him all weekend and conclusion was as long as he is pain free he can go for walks, nothing crazy though. (the boy does NOT do house arrest AT ALL)

She thought possibly seizures which the vet is keeping an open mind about

THinking is if it was linked to his dyscospondilitis he would be continually in pain and certainly wouldn’t be able to do his hydrotherapy?

Any thoughts appreciated as they are a bit flummoxed until the MRI

Poor boy
I'm sure the vet give him a thorough examination,just wondering if its possible he could have knocked himself against something?Maybe even fell and hit his head.
Mine has a "blind side" actually the other side isn't much better but occasionally she does bump against things.
Just a thought
Fingers crossed for him,really hope he's going to be ok xx