"Clearing" is applied when student's result cannot meet the requirement of the Firm and Insurance Choice, or student wants to apply for other choices after result release (in that case students have to reject the Firm and Insurance Choices).
Yes, need the DSE result. I suggest asking your friend (and son) go to those University Fair in HK, e.g. Amber, and do the clearing there.
https://www.amberedu.hk/?gclid=CjwKCAjw2K6lBhBXEiwA5RjtCcV8Kp-DKecL9m4_2HQOxin5BvUHUN9S66zsSjUGdxS9h7_l_TFE7RoCGrIQAvD_BwE