Has he always done this or is this recent behaviour? If the latter, a urinary infection or some other problem is the likely cause. Either way, I'd have him checked out by the vet.
If it isn't a urinary problem - which only the vet can rule out - then maybe he was never properly trained. Have you tried putting down pee pads? Some people never take their little dogs outside but train them to go inside pretty much exclusively.
I would take him out yourself as often as possible, rather than waiting for him to make a mistake. When you are in the house, the best thing to do is tie him to you, so you can catch him before he pees. It sounds like work - and is - but I wouldn't assume that just because he is a senior he isn't capable of learning that going outside gets him special treats and loads of praise.
You are an
for sticking with this old boy, many others would have rushed him into the death chamber.