Man I just found this, maybe a WHOLE YEAR LATE. But nevertheless I'll give it a shot if ya still on it.
If your dog is that old...from nwo on I'll refer to him as pup coz mentally they never get old

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I do not know what might have happended to him before. Since he is blind, any blind creature will feel fear when not constantly in company of somebody, more so if he is also deaf which is extremely sad.
Problem with your dog...or not problem, issue, is that he might have been abbused or attacked by a person or other dogs thus the slightest hint that he is alone will induse fear or kick in the post traumatic stress disorder (yes it not only happens to human soldiers) and recolect the fear of pain he suffered before.
The only "theory" that I heard once in a dog show, was that if you adopt a kind of dog that is deprived of its 2nd best sense, hearing, they need constant company, more so if blind, and thus a dog that is not only deaf, and blind, needs to sleep next to somebody, to feel the touch and company of someone constantly 24/7.
A friend in a dog show had a blind pup, yet not deaf, the only way he fixed the pup's fear was letting him sleep on his bed every night...more or less the pup slept to the right of my pal and he hugged him every night.....teddy bear style.
And until now it works wonders.
Lastly, if you adopt a pup you have to take into account that 'adopting' is VERY different from buying from a litter.
YOU DOT NOT know, NOR can you even begin to imagine that happened to that pup. They might be very sad because they had owners and the owners left him alone for some reason, and since animals have a sense of loyalty humans can barely understand that is extremely traumatic.
And so if you choose to have a pet, adopting it, but even more with certain issues to his health or problems with his body, it takes a VERY long time to somewhat mend wounds. on top of this, he is 13!, which means he has suffered for an undetermined amount of time and thus he mental wounds can be severe.
There is the chance it might take 6 months of constant company, it can be a year, or it can be he will remain in a constant state of fear for the remainder of his days.
Nobody can tell how long will it take, but adopting an animal is not something you do out of compassion only, you have to take into account the animal your getting might have a memory that will never be healed, and I would be not only dissapointed by extremely angry at those that adopt an animal and then leave it because the pup's problems wont heal.