If you have his enlistment date or a time frame, one option would be to check the home town newspapers of where he lived when he enlisted. Units of assignment were often published for some of these "home town" boys. Another option is to check with the town and/or county clecks office. Ask for any VA or Veterns records they may have that pertain to your Grand Father.
If you haven't done so already, make digital scans of all the documents you have for your family archives, preferably in PDF format. Additionally, if you have a good photo processing program, you can scan in a JPG format and try and enhance the information on the document. Sometimes converting to black and white and doing a negative can make it more readable.
I believe you also mentioned a photo of him uniform. Does it show any patches or insignia...? If you can identify an insignia, compare it to the ones on the chart at this link;
I use Photoshop Elements version 9 which is a very good program. If you would like for me to look at your documents and photo and see what I can do...you can send to my email by clicking on my name above in blue.