Do you know where and when your grandfather was wounded. Do you have his discharge or separation papers which would give most of the pertenant details about his service? His separation paper would list the Division and Regiment he served and the campaigns included in his WW-I Victory Medal.
Do you have his WW-I Victory Medal?
Purple heart was not issued until 1932. At that time veterans could apply for the PH if they were authorized to wear the Wound Chevron.
You might be able to find a wound ribbon from a military supplier or antique dealer. You also could copy the image shown on Wikipedia.