Concerning his Son (peri tou huiou autou). Just as Jesus found himself in the O.T. (Luk 24:27, Luk 24:46). The deity of Christ here stated.
According to the flesh (kata sarka). His real humanity alongside of his real deity. For the descent from David see Mat 1:1, Mat 1:6, Mat 1:20; Luk 1:27; Joh 7:42; Act 13:23, etc.