His first son, Jan de Witte (the Younger), after completing schooling in Jesuits' college, joined the ranks of Polish royal artillery corps in February 1726, in which he later served until his death. Being promoted several times (captain in 1735, major in 1751, lieutenant-colonel in 1754, colonel of the artillery in 1762, major-general in 1767), he finally became lieutenant general in 1781, from 1768 being also appointed as the commander of the mentioned Kamieniec fortress and other minor frontier forts in Podolia simultaneously. In 1735 he married Marianna Lubońska who bore him numerous offspring. His career's supporter and promoter was a powerful Polish noble, Grand Crown Hetman Wacław Rzewuski. In his later years he also attracted the attention of King Stanisław August Poniatowski, who, in an act of recognition of his loyal duty, highly rated knowledge and works, awarded him with the Order of St. Stanislaus in 1781. It was Rzewuski who pronounced a recognition of de Witte family's "indygenat" (the recognition of foreign status as a noble) in 1764, four years later positively approved by the Polish Sejm. Jan de Witte was considered as an authority not only in the art of war, but also in questions concerning the Near East, as he was one of the best-informed specialists in Polish-Turkish affairs. For example, he kept a wide correspondence with the Turkish pashas from Ottoman frontier forts along the Commonwealth's south-eastern border. As a military man he enjoyed a reputation of a rational, but also exceptionally demanding and severe officer, especially among those under his command, which resulted in incapability of gaining affection among his subordinates and goodwill among the military command (for instance, his abilities were often questioned by Alojzy von Brühl, chief general of Polish royal artillery corps). During the course of his military career he managed to amass a small fortune, including the rural estate at Sawiń. De Witte was a passionate collector of coins and medallions, also a great book-lover.