Stopping a puppy from chewing on foreign materials in the yard is an impossible task. Fortunately given the variety of objects that puppies pick up the frequency of significant problems is very low. Being watchful and ready is your best defense. Ornamental plants can pose a significant threat due to toxicity. Some Lilies are toxic and there are many other common plants that pose a threat. Astro turf can be difficult to clean. I would suggest some type of exercise pen to keep him away from danger.
There is a doctor pup article on common toxic plants.