As cats age, they do lose their eye sight.  I have had geriatric cats
who were blind and did NOT act this way at all.  My vet always said
that as long as you don't move furniture around, etc. a blind cat can
function just fine.  Maybe geriatric cats lose their sight slowly, so
they adjust?

I know you said he doesn't have hyperthyroid or high blood pressure, but I
wanted to send this blurb:

MrR- Sudden onset blindness due to hypertension is a well recognized
syndrome in cats. It can occur due to kidney failure or hyperthyroidism. If
treatment is started within a day or so of onset there is a 50% chance of
vision being regained according to Dr. Norsworthy, writing in the June 1996
Vet Forum magazine. He recommended using amlodipine (Norvasc Rx) for
treatment. Diltiazem has a similar method of action (I am pretty sure both
drugs are calcium channel blockers) so it may work as well. Your vet may
have access to information on this that I do not.
Hyperthryoidism is usually easy to test for. Most affected cats have high
thyroxine (T4) levels in their bloodstreams. Some cats have this disorder
despite normal T4 levels, though. In these cases, a T3 suppression test may
be able to differentiate between affected cats and those whose T4 levels
really are "normal". It is always wise to run a complete blood panel when
checking for hyperthyroidism since it affects other organs, including the
kidneys.

Blind cats usually do fine. The sudden blindness is a problem as it doesn't
allow much adjustment time for the cat but usually they adapt well, anyway.
I can only remember one or two pets in the 18 years I have been practicing
who really had great difficulty adjusting to blindness. Of course, some
things do have to change. Outdoor cats need to become indoor cats and it
isn't a good idea to rearrange the furniture frequently. Put food bowls in
the same place everyday. Make sure litterpans are easily accessible and in
low traffic areas so they aren't likely to be blocked by anything your cat
could bump into.