Based on your information I would venture to say that your grand dad was a member of the Bavarian 10. Infanterie-Division, specifically its artillery regiment, Artillerie-Regiment 10, which was garrisoned at Amberg where as a recruit he most likely received his first training as a Kanonier. The main reason for my saying so lies in your description of his rank as that of Wachtmeister. That particular designation, which in all other branches of the Heer was that of Feldwebel, was used only in cavalry and originally horse-drawn artillery units of the German Army. You will find information about the 10.ID on this website, the Forum der Wehrmacht, and the Axis History Forum.
To become an Unteroffizier or Wachtmeister, a man would first have to be recommended for promotion by his superior officer, followed up by an order to attend and pass a number of special leadership courses at a designated school for NCO candidates.There were many of these Unteroffiziersschulen throughout Germany before and during the war.