I used to work with old people and found that drugs impaired their quality of life. They made them confused and restless, and it could be the same for your dog. I really don't know of any drugs that help with senility, so all you can do is love him for the joy, happiness, love and loyalty he has given you all his life him and make sure he is as comfortable as his tired body allows.
My old Jack Russell had cataracts, was deaf, had fits and was incontinent when he woke suddenly, but just went on loving life.He died at 17 - I had had him since he was three weeks old.
Thinking of you.