You can get at some basic public information via military.com's BuddyFinder.
My grandfather (who fought in WWII in the 29th Division) is listed on the Buddy Finder even though he never registered with the site. It gives his branch, rank, specialization, and state of residence. That is likely all you will be able to get online unless someone has randomly posted something about him on the internet.
A few years ago, I helped my grandfather request his war records and other information from Department of Defense. As a previous poster mentioned, most of the war records were destroyed by fire at the national storage center, however, there is a significant amount still available. We were able to receive his discharge papers, which included his entry and exit date, his decorations, the battles in which he had participated, etc. The DoD also sent us replacement medals for his decorations because he had lost them over the years (he had done his best to forget WWII until late in his life).
If I recall correctly, the records may only be requested by the veteran (if he is still alive, N/A in your case), the veteran's spouse, or (I think) someone in the immediate family (e.g., your mother may be able to request them because it was her brother but you may not because you are a niece). There are definitely restrictions on that stuff. It's not all necessarily subject to FOIA. That's one reason I tried to get my grandfather's stuff taken care of while he was still alive, because otherwise it would have been very difficult for me to get the records. They also only sent replacement medals because he was still alive at the time, so don't get your hopes up on that.
Anyway, I hope that information is helpful.