Washroom behaviour

Question
I have a 10 month old choc. lab and I put him on a rope outside to go to the washroom;  since the rope is a certain length, he only has a certain radius that he can use in the yard.  In the last couple of days I've noticed that he won't even step on the grass let alone go to the washroom on it.  I've heard that skunk's urine can cause this behaviour;  is this true or could it be something else?  How can I get him to use it again?  I've tried to force him, lure him with treats/toys, cut the grass, raked it, and soaked it with a hose to try and wash the smell away, but nothing worked.  Please help.  Thanks!!

Answer
I never heard of a problem with skunk urine.  

Make him exercise.  Throw a stick or a ball for him to chase.  Just be careful not to throw it out of his reach.  The exercise will stimulate his body to where he can't hold it any longer.  You could also try walking him on leash elsewhere.