It definitely sound like some type of brain injury. And the vet is right, no way to really find out without an mri or ct scan.
I wouldn't put him to sleep unless he seems painful, or his quality of life is poor.
We had a dog that got kicked in the head by a horse. He didn't go blind, but did do the walking in circles. It took him a couple of months to get better.
My sister's 16 year old kitty had a stroke and went blind temporarily. She did get better and didn't have another stroke until she was 18. That on killed her
I think just keeping a close watch on him is the best course of action. Whatever you do, do not let him outside. He is not safe out there anymore
for your sweet boy.