How do I guarantee that my son retains residency in Greece as the dependent of a Golden Visa holder even after he turns 21?

As per the existing legislation in force, your son is entitled for a residence permit as your family member till 21 years old and subsequently for a 3 years independent residence permit of his own, till 24 years old, which can be renewed under conditions. Under these conditions the best option is him to be the main investor and you as his parents to be included in his application therefore. Furthermore, there are related provisions for this issue in the already voted New Immigration Law, applicable from 2024. When ready to proceed with your investment better to check this issue again.

You have to donate the apartment to him. He will be the main applicant and you will get residence as his parent.