What you need to do is get a copy of his DD-214 form, which will tell you all about his military career. You apply for it using information found at:
http://www.archives.gov/veterans/military-service-records/Not many of the World War I records were destroyed in the big fire, so they may be able to help you out.
The fact that you have his service number and name should help them find him.
Good luck!
