Jamieson, Fausset and Brown on John 16:8

8. And when he is come, he will, &c.--This is one of the passages most pregnant with thought in the profound discourses of Christ; with a few great strokes depicting all and every part of the ministry of the Holy Ghost in the world--His operation with reference to individuals as well as the mass, on believers and unbelievers alike [Olshausen].

he will reprove--This is too weak a word to express what is meant. Reproof is indeed implied in the term employed, and doubtless the word begins with it. But convict or convince is the thing intended; and as the one expresses the work of the Spirit on the unbelieving portion of mankind, and the other on the believing, it is better not to restrict it to either.