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If the rash is in an area that comes in contact with grass then it is possibly a contact allergy to grass. It could also be contact allergy to something else in his environment or not an allergy at all. I recommend watching it closely for changes including spreading, development of rashes elsewhere on the body, pustules (little things that look like pimples), or licking at the area then it should be evaluated by a veterinarian to make sure there is not something else going on.
You can also try a short course of Benadryl to see if this helps. If so, then it was likely an allergy. The Benadryl can make him sleepy. His dose is 1-2 mg per pound. You can use the liquid or tablets. If using the liquid make sure that is does not contain xylitol as this is toxic to dogs and can cause liver failure.