You should see if you have a local library that has access to ancestry.com. Last year, I found out that Okla History Center has library version to ancestry.com. I looked several of my relatives and was able to print out their draft registrations information from WW1 and WW2. The WW1 information is microfilm copy of the actual record. Most of the WW 2 is a copy of the actual document but does give you some information. Such as branch.
It would be helpful if you knew what town or county he lived in during the time of his draft or enlistment into WW2.