Resting upon him represents being with him and helping him. The word "him" refers to the one who would become king. (See: [[rc://en/ta/man/jit/figs-metaphor]])
Here the word "Spirit" refers to an ability or quality that the Spirit of Yahweh would give him. Alternate translation: "and he will cause him to have wisdom and understanding, instruction and might, knowledge and the fear of Yahweh" (See: [[rc://en/ta/man/jit/figs-metonymy]])