The upper patch on his shoulder is the patch for 85th Infantry "Custer Division", see here: http://www.custermen.com/ The lower patch are his Sergeant stripes. I don't know what the ribbon is for, might be a bit tricky to work out without knowing the colors! I'm guessing that you may be able to find out if you track the combat he as in.

The jeep is a Willys-Overland jeep. Willys made budget cars before the war and moved to the production of the jeep you see in the picture as the war came along (they were demonstrating the vehicle as early as 1941). It was a good design, rugged and capable but also easy to maintain and repair. The Jeep brand still exists today and can be traced back to these vehicles at the start of WW2. I can't track down the marking on the back fender, maybe someone will have a reference book of them.

Hi, the shoulder patch is that of the US 85th Infantry Division (aka the 'Custer division').

Among other actions, they played a key role in the relief of the ANZIO beachhead and the liberation of Rome.

http://www.custermen.com/History85.htm

More online reading:

http://www.history.army.mil/html/bookshelves/resmat/ww2eamet.html

The stamp from the U.S.S. Texas is interesting. It looks like it says "FEB 7 1939". For modern letters that would be the date it was mailed, but maybe that is just the date the rubber stamp was made? According to Wikipedia the Texas was in Italy between July 27th and August 11th 1944. So I wonder if the letter was written around that time?

Someone else already posted this, but I think you can get an official service record from the Military. I am not sure what level of detail that provides, or who you have to be to do the request. Looks like you might start here. Good luck.

If you are looking for information on the ribbons, maybe try /r/military these guys are incredibly good at decyphering them. May prove tricky because of the black and white but I would give it a try.