"Son of" is an idiom to be like something. It can be translated as 1) "you child of the devil" or 2) "you are like the devil" or 3) "you act like the devil." (See: [[rc://en/ta/man/translate/figs-idiom]])
Paul is grouping Elymas with the devil. Just as the devil is an enemy of God and is against righteousness, so was Elymas against God and his righteousness.
Paul is rebuking the magician for following the devil. This can be stated as "You must stop saying that the truth about the Lord God is false" (UDB). (See: [[rc://en/ta/man/translate/figs-rquestion]]).
This is an expression for "what is true about the Lord." Paul was rebuking the magician for calling what was true about the Lord lies. (See: [[rc://en/ta/man/translate/figs-idiom]])