I didn't know your father, but I can provide a little information on this battalion, and perhaps shed some light on his military history.
The unit was activated at Fort Bragg, NC, 4 Mar 43; after training, unit departed NY P/E, 6 Apr 44; arrived England, 16 Apr 44; arrived France-ETO, 26 Jun 44 (D-Day, plus 20 days); arrived Boston P/E, 16 Dec 45; inactivated at Camp Myles Standish, MASS, 17 Dec 45.
The 793rd FA BN was non-divisional, meaning that it more or less floated from Army or Corps on an as-needed basis, where it manned an 8-inch tractor drawn howitzer.
Normandy , 6 Jun 44 / 24 July 44 Northern France, 25 Jul 44 / 14 Sep 44 Rhineland, 15 Sep 44 / 11 May 45 Ardennes-Alsace (Battle of the Bulge), 16 Dec 44 /24 Jan 45 Central Europe, 22Mar 45 / 11 May 45.
Location at war's end, 15 Aug 45: Wiesack, Germany.
You know that the number you refer to is his army serial number.
Btry B is for Battery B, which would have been one of four batteries (Companies in infantry lingo) making up 793 FA BN. They were enumerated A-B-C-D.
APO was the fixed postal identification of his Post Offoce.