You could try letting him out only when you can watch him and either direct him to go in a more appropriate spot (this may require you to walk him on a leash in your yard) or water the grass heavily after he has gone to dilute the urine. This of course would take a great deal of time and effort on your part. Instead, perhaps you can designate a non-grassy area of your yard for your dog to urinate. If you don't have such an area, it might be worth it to add a patch of gravel and train him to go there.
Dog urine burns grass because of the high nitrogen level (salts and ammonia can be an issue as well). Nitrogen in urine comes from protein in the diet. You could try changing your dog's diet to a food with less protein. Look at the ingredients on the bag/can to determine if protein is a primary ingredient in what you're feeding. There are a lot of dog foods that list corn or other starches as the main ingredient. Just be aware that if your dog is prone to GI upset or allergies, a dietary change can cause a flare up so make the switch gradual. (Fyi, I've yet to hear any good feedback about the "supplements" you can give dogs for this problem and I've been a vet tech for 9 yrs. They wouldn't be my first choice.)
Most costly but most effective would probably be to contact a trainer. Even if your dog is obedient, a trainer can help with behavior modification. http://www.barkbusters.com/
Best wishes! :)