B.M.
What type of grass do you have on your lawn? There are certain types of grass, like Kentucky Blue Grass and Bermuda Grass that can cause more sensitivity. And dont set him in the grass if its recently been mown, and make sure you have a bag for your lawn mower so grass clippings arent left on the ground.
But, just as a side note, my daughter did this, my sisters did this and most small kids I know did this. By the next summer they all grew out of it. It doesnt hurt to bring it up with your pediatrician however, especially if allergies run in your family.