Toys represent prey. Chewing on a toy - means killing a prey. A wolf, or a fox does chewing otions with his jaws to "finish" a mouse or a bird, so does your dog when catches his toy. Your trouble is that nothing comes after, he doesn't know what else to do with it, that's why he continues to chew. Or - he doesn't want to compete with you.
A decoy is a big and dangerous bull, who doesn't want to allow the dog to rip meat off his bones so easily. But, the dog happens to be stronger and always wins, getting his piece of meat (that is the sleeve, decoy sheds it for him as a reward). Some people look at the prey work (bite work) as it is a combat between two beasts - man and dog - want to be in posession of prey object (sleeve). I wouldn't say anything about defence here, because it introduced in training after the main interest was ignited. In all of this one thing should be important to you - the dog wants to deprive someone from something and that someone doesn't want to give it away.
Start teasing your dog, running away from him with the toy, hang it on a tree, throw it and fetch it yourself, continue to run, don't make it easy to grab for him. He would enjoy more to be in posession of it. Then, you need another person in order to break already established chain of actions, i.e. fetching and chewing. You and your dog should be on one end, and that person on the other. Have your dog on lead, throw the toy, and ask that person to fetch the toy befor your dog gets it, tease him and play tug, but not for long and not really tugging, just holding to it. With the next throw let your dog fetch the toy, and that person to try removing it from him by holding and pulling gently. The next time he or she shold pull the toy with one hand and stroke your dog with the second as a distraction, your dog should hold. Try to be inactive yourself, as your dog is on the lead, during this game of three, except introducing a new command to hold the prey toy. As for the command, it should sound intriguing, you can simply choose some hissing sounds for it "Sh-shoosh-sh-sh!" or some abstract hissing word of your choice. Play this game of three with a ball-on-rope and train outing with immediate reengagement.