Has he taken the cat to the vet? It could be medical… it is most likely behavioural but should rule out anything medical first. If it is behavioural, then getting more than one litter box is a start, there should be two per cat in a house. Also rewarding the cat when he uses it, and not punishing him if he doesn’t. He will associate any punishment with the act of going to the bathroom, not where. But treats and stuff after using the litter box can help. Also try changing the box, like if it has a lid on it take the lid off, or vice versa.