What you can do is to take a jump rope, or a piece of rope, and make a large circle. Keeping him on a loose leash, take him there everytime to use the bathroom (and praise when he does!). If he attempts to use any area outside the circle, correct him and bring him back to the circle. This also makes clean up easier. After a while of doing this, test him..let him off to use the bathroom and if he goes to the circle, immediate praise and a treat. If he goes to another part of the yard, bring him to the circle. He'll catch on eventually.

There are also products available to spray, and to keep animals away...let me see..
Off Limits Spray
Scarecrow Motion Detection Sprinkler
Or what about just a small picket fence of some sort? Or an invisible containment system-it would be safe both for your lawn and your dog.

Let us know how it goes!