Paul talks of the Spirit as a separate entity of the Trinity. He uses “he” when refering to the Spirit, not “it.”
I beg to differ on both accounts. St. Paul speaks of the Spirit of God and not of a separate Trinitarian Hypostasis, i.e. God (or Lord, vasileos) the Holy Spirit. In St. Paul's case, the Spirit is subordained to God (the Father), not co-substantial with Him. St. Paul's concept of the Holy Spirit is perfectly Judaic, being God's energy rather than a Divine Hypostasis.
Christians eventually developed the concept of grace that represents God uncreated enegries, as the understanding of the Trinitarian formula evolved and the Holy Spirit was recognized as a Divine Hypostasis and God Himself that estows grace.
The Greek terms for "self" is autos. Depending on the Greek gender of the word (which the English language doesn't have), the appropriate pronoun will be he/she/it. Some English-bible translations use "it" others "he" (i.e. NIV) when referring to the Holy Spirit, which is grammatically correct.
There is nothing in St. Paul's writing to indicate that he speaks of the Holy Spirit as a "he" because the Greek word pneuma (spirit) is of neutral gender (neither masculine nor feminine), so the appropriate pronoun is it.
The fact that it appears as a "he" in some English translations shows that some have taken it upon themselves to "correct" the infallible word of God so as to make it fit out theology.