Yes, there is a difference between Staff Sergeant and Sergeant.A SSGT is one rank higher than a SGT.Here's a link to a page which shows the different Army ranks and the patches they wore.The pics show modern rank insignia, but the general design hasn't changed since WWII: http://usmilitary.about.com/library/milinfo/rankchart/blenlistedrank.htmhttp://usmilitary.about.com/library/milinfo/rankchart/blenlistedrank.htm
Regarding specifics on his wounds, see message 10907 on this board for info on how to obtain govt. records.I'd try the VA first to get his medical records (as next of kin they're available to you). Try an internet search of his unit to find a history of their actions during the war.
It's just a matter of putting the pieces together.If you know the date he was wounded and his unit, you can find out what battle they were in on that day, where they were fighting, etc. It may be difficult to find the EXACT circumstances regarding how he got wounded.With literally hundreds of thousands of casualties during the war, that type of info wasn't kept.
If you contact me by email I'd be more than happy to help you with it.Or simply post his info to this board-- I'm sure you'll get lots of responses.