He shall cry unto me, Thou art my Father - He shall appeal to me, or come to me as a Father, and as his only hope and defense.
My God - He shall come to me as God, and shall recognize me as his God, his only trust and hope.
And the rock of my salvation - See the notes at Psalms 18:2. The meaning of all this is, that he would at all times recognize him as his only trust and hope, and that he would be faithful on his part to God.