How can i find the family of a WWI vet?

I recently accepted a new position as a contractor at an Army base and when i was going through my desk i found a dog tag belonging to someone from WWI. His name is Robert Phillips and his serial number on the back is 6884649. If i could post a picture i would. All i want to do is find his records so i have a good basis to find his family and give them this piece of their families history. Ive tried websites that want me to pay $50 a year and other sites that are free just lead to more sites that lead to more sites with little to no help. Im a veteran myself and respect those that have served, any ones help would be greatly appreciated thank you!