There are several possibilities of what may be going on with your boy. I will address each one. The first issues is called a Vestibular Disorder. This is an area in the middle and inner ear that controls the balance center and helps all living creature to be able to walk without falling. Some of the signs are walking like drunk. eyes darting to one side or the other, head tilt to one side, circling when walking and some other symptoms that may come with this problem. In older dogs Vestibular disturbance is usually idopathic and can come on suddenly and last about 14 days and then clear as quickly as it started. I would like to supply some further information about Idopathic Vestibular Disturbance.
The next issue could be a massive seizure that has affected the dog. In older dogs it is very hard for them to come out of a seizure and they can have Cluster seizures that can happen one right after the other. You can read about seizures here: http://www.canine-epilepsy-guardian-angels.com/site_map.htm
A Brain tumor in older dogs can also cause the symptoms you are seeing. It can affect the dog The lack of recognition and the Vestibular symptoms you are seeing. Please read through this link and scroll to symptoms at the bottom: http://cvm.ncsu.edu/vhc/tc/clinical_services/neuro/brain_tumor.html
The last thing that you had mentioned is a Stroke, but is actually very rare in dogs, however you can read more about Canine strokes here: http://dawn-m-smith.suite101.com/understanding-canine-stroke-syndrome-a76632
Since this has happened, your dog should see the Vet asap as this may be treatable if it is Vestibular or seizure activity. The sooner the dog is seen the better. Please let me know if I can be of further help. Joan
This may be a Brain tumor as I said, but the Vet doing a Neurological Exam would be able to best address what is actually happening. Ther are some many symptoms that mimic each other, only a hands on exam will let us know what is happening. At 16 he is an old man, so it is hard to specifically pinpoint what has casued it. Joan