The sooner you can have him neutered, the better. Once the behavior is well established, it is harder to break.
If he has begun to mark, neutering alone might not stop the habit. Limit his freedom of movement in your home, supervise closely, and give him a firm "no" when you see him try to mark.
Always praise profusely when he potties in the correct place -- outdoors or pad. This will help stop the marking.
Some people also advocate using belly bands. |