I agree that a trip to the vet is probably warrented due to his age. I also wanted to add though from a behavioural POV tat I was reading an interesting journal on dog urine marking and their definition of marking vs urinating is that sniffing preceeds a mark. I thought that was quite interesting. Perhaps it is more behavioural - again this could be age or could be something like changes in the household, new neighbours etc?
But the first step would definitely by the vet anyway to rule out health problems. Good luck and keep us updated.