1.He's most likely peeing there because of the smell of food omitting from the grill as well as the smell of other animals being drawn to that grill. He's laying out his sent to demonstrate to the other animals that this is HIS space. Your current methods have taught him to only spray when you are not looking.

2.You may need to step up and act like the alpha, claim your 'right' to the grill. I would start off cleaning the grill and area with a Ammonia Vinegar mix spray (remember to keep Ammonia away from Bleach, never mix the two). Let this become 'your' sent. (White vinegar makes a great cleaning solution so I would also use this to clean other parts of the house, to lay out 'your' scent). I would make sure you clean the grill often.

3.I have a mixture of 3 parts vinigar 1 part water 10 drops of tobasco made up to deter my dog. Cover the intier thing in it until she gets the dift.

4.Your response will have to be two-fold. First, you need to GET RID of the odor. That isn't always easy, as dogs have very powerful noses. For light saturation, spray-soaking rubbing alcohol and lysol on the spot (get it good and wet, then let it dry)

5.Enzymes
If he keeps going back to the same places, he's probably still able to smell the urine. You need to use a special, enzyme based cleanser, which you can get at any pet store. Dogs will gravitate towards the smell of urine, which is hard to cover up. Until the enzyme cleaner is used, the smell will remain, whether a human nose can tell, or not.