How would I address Mok Souw Leung

You can call him Mr. Leung.

You gotta find out what's the family name/surname. Both Mok and Leung are common Chinese surname. In this name, to me "Mok" sounds like the family name, the family name is placed before the given name; however, sometimes in the Western society, Chinese would place the family name at the back because in Western culture Family name = last name. For eg., Lee Jun Fan is written as Jun Fan Lee (btw that's Bruce Lee's name)
Besides, Souw Leung sounds like the given name, Mok Souw can be the given name but it sound rather ancient and uncommon. If that's true, it should be Mr Mok.
p.s. the pronunciation of the name sound Cantonese.