His father also called him Loong (not Hsien Loong), at least sometimes:
"He was still young and it was better that someone else succeed me as prime minister. Then, were Loong to make the grade later, it would be clear that he made it on his own merit."
"He was still young and it was better that someone else succeed me as prime minister. Then, were Loong to make the grade later, it would be clear that he made it on his own merit."
http://www.singapore-window.org/sw04/040531a1.htm
(They use English in the family, although Lee Hsien Loong can speak Chinese fairly well.)