Is he desexed? That would be the first point if he isn't. He definitely is spraying (e.g. he doesn't squat but backs up to a surface). You need to clean it with a suitable product that doesn't contain ammonia as that mimics the smell. I think you may be able to get something from the vet - we used to stock it but I cant remember the product name. Then you can try a product like feliway which has a pheromone smell similar to what comes off when they rub their neck. You cant smell it but they can and it can help reduce spraying. There are also behavioural thing you can try. He is marking so if you reduce his vision to where other cats may roam (e.g. windows) that can help. If it is a particular spot you can place alfoil where he would stand, or citronella on the item as a deterrent.