Toy-suckling is similar to human babies being fixated on a certain blanket. In infants and toddlers, the term "security blanket" often arises because the child seems to feel more secure with the blanket or object. Some dogs are more prone to object-suckling, including those who left their mother too early or those who have been re-homed several times from a young age. It's also more common in sensitive dogs prone to anxieties than in dogs with stronger characters. Some behaviorists recommend leaving it; if it doesn't do any harm, then why change it?
Take notes of when your dog is suckling and what's going on around him. If it's stress-related, your notes could help you pinpoint a trigger. Some common stress triggers include new people, being punished, loud noises and car rides.
Talk to your veterinarian before trying to stop the behavior on your own, especially if the suckling has reached a compulsive level. Some medical issues are the backing factor to many compulsive behaviors.
Substitute a new toy or a game when your dog is suckling to see if it helps pull him out of the behavior. Offer a walk, a game of tag in the yard or a new chew toy that isn't soft. This is the same type of training recommended when puppies or dogs chew on stuff they're not supposed to. Take away the object he's fixated on and then offer the new one in a fun, energetic and encouraging tone.
Spray a taste deterrent on the item to keep your pooch from suckling it.
Let him be. The reasoning behind suckling isn't completely understood, but some veterinary behaviorists, such as Gwen Bailey of DogBehaviour.com, believe it is a comforting measure employed by dogs of all ages. Taking the item away and not letting him have the comfort of his "security blanket" could cause stress on your beloved pooch.