Can my adult South African son join me here in UK. I am naturalised British Citizen of 5 years.
Unlikely. Your son only qualifies for a family visa based on his relationship to you if he is under 18 or in need of long-term care. From https://www.gov.uk/join-family-in-uk/eligibility:
Joining your parents
You must be under 18 and all of the following must apply:
Coming to be cared for - you’re an adult dependent relative
You must be dependent on a parent, grandchild, brother, sister, son or daughter who is living permanently in the UK.
You must prove that:
- you need long-term care to do everyday personal and household tasks because of illness, disability or your age