Senility is a possibility, but try these too:
He may have a bladder/urine tract infection or stones. It's going to make him feel like he's got to pee, but he can't.
Or there could be an upstart out there, and he feels he's got to mark his territory constantly (they'll do that even if neutered...I call it 'invisible ink')
The dining table is because it's high. He feels safer. He can see trouble heading his way in plenty of time. Since he's old, he needs the extra time. Is there someplace up a bit higher you can put his bed? Or sprinkle some cat nip in it. Mine goes bonkers for the nip then passes out into a zombie like state right on top of the nip.
Of course, he's could also just be being a cat and winding you up.
If he's straining to pee, or if you notice blood in the pee, whip him off to the vet quickly. He's in big pain, and it can be treated fairly easily.
If not, just give him more (soothing) attention than you normally do, and he'll probably settle back down.
I've turned into a bit of a 'cat's doorman' lately, but it's because I've changed my (our) routine.
If he still insists on the dining table, put a lazy suzan (turntable) on it where he sleeps, so at least when you eat you don't have to stare at his bum!