At this poor baby's age, I'd take him to the vet for a thorough check-up.

Old dogs are much like old people;
sometimes these things indicate a treatable, correctable underlying problem, & sometimes the problem can't be corrected, & you just have to put down pee pads or put him into a contained area overnight.

(Spanking a dog is just mean anyway, as well as being counter-productive;
if you catch a young dog *in the act*, you can thunder "NO!" & show him the alternative (pee pad, out the doggie door, etc), but hitting your dog teaches him to fear you.)

I wish both you and your old guy the very best.

I totally agree with Sylvia. With old dogs you just have to deal with their problems the best you can. If he's been housebroken all his life, he wouldn't do that if he could help it, so by all means do NOT punish him for it.

If the vet doesn't find anything, it could be that he's developed a food allergy. You might want to try a different protein source than what he usually eats, even going to something less common like buffalo, or fish. Make food changes very gradually, though, mixing increasing amounts into his regular food until you've made the switch.

This is the very reason why my dogs sleep in crates their whole lives. It makes it so much easier to deal with these kinds of problems which so often happen in old age.