Hi there and thank you for your question,
Unfortunately no, as he is now an adult.
Adoption is a legal way for an adult single person or couple to become the legal parents of a child.
According to the Children's Act 38 of 2005, a child is considered someone under the age of 18 years and when it comes to the adoption process, anyone older then 18 cannot be adopted.
You can read a little more on the process of adoption in South Africa here: http://www.bregmans.co.za/adoption/
What you should do, and so he feels part of your family, is to make provision for him in your Will, and perhaps he can legally change his surname to yours. In that way, while he won't be your legally adopted child, he will still feel that sort of connection.
As an adult, he just can't be adopted. I'm sorry that this is probably not the answer you want, but I can only advise on the law.
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Please note: This answer does not create an attorney-client relationship. Please consult a local attorney in person for legal advice. This information is being provided so you can better discuss legal matters with your attorney.