The first question to ask is whether or not he's house trained. Is he neutered? Does he have separation anxiety? How much time does he spend in the house at a stretch before being let outside to urinate? Have there been any recent big changes in the household? How long has he been part of the family? Has he been to the vet to rule out health factors? Determining the source of the problem will go a long way to finding the right course for correction.