Answers
Following the provisions of Greek immigration law 4251/2014, the children of third-country nationals who are holders of a permanent resident permit as property owners are granted a resident permit for family reunification for up to 21 years. After the 21st year, they can be renewed for a further three years with an independent resident permit of up to 24 years of age. After the completion of 24 years, your son has alternative options to follow in order not to lose his resident permit. He could apply for Greek citizenship under the conditions that will be in force, or to become the main investor of your investment through sale or parental donation. In such a case, you will keep your resident permits as his family members too.
A Greek Golden Visa includes dependent children up to age 24 (initially the age of a child shouldn't exceed 21, then s/he can extend unitl the 24th year of age). As now your son is 16 years old, at the age of 23 he could apply for Greek citizenship after seven years of residency in Greece.
Please be aware that children of third-country nationals who have been accepted in Greece under the terms and conditions of residence permits for property owners are granted a residence permit for family reunification for up to 21 years. After the 21st year, they can be renewed for a further three years with a resident permit of up to 24 years of age and then renewed in accordance with the provisions of immigration law.
Our suggestion is that you sell the property to your son after he turns 18 years old.