The Simple Answer is only the NKJV uses "He" rather than "he". They also allow "he" in their footnote, though they seem predisposed to "He".
In response to your question: the NKJV is the brainchild of Dr. Arthur Farstad of the Dallas Theological Seminary, which is noted for it's Literal interpretation of the text(called Complete Equivalence as opposed to Dynamic Equivalence) which other translations relied upon. He is also Dispensational in his eschatology, having also authored the New Scofield Study Bible.
Dispensationalists take the view that the "He" is the Holy Spirit. In an commentary on 2 Thess. 2:7, Dr. Dwight Pentecost, also of Dallas Theological Seminary says,
"The Restrainer is referred to both in the neuter (what) and masculine
(he) gender. This mix of gender appears in relation to the Holy Spirit
Who is a person, but also described using a Greek term which is neuter
in gender (πνευμα [pneuma] ). It is also said that the Restrainer “now
restrains” and will continue to do so until “He is taken out of the
way.” Since the man of sin has yet to be revealed,1 we can infer that
the Restrainer, whoever or whatever he is, has been effectively
suppressing the revelation of the man of sin for over 2000 years. When
we collect the pieces of evidence concerning the identity of the
Restrainer, we find: The Restrainer is referred to as both neuter (τὸ
κατέχον [to katechon] , “what is restraining”) and masculine (ὁ
κατέχων [ho katechōn] , “He who now restrains”). The Restrainer
existed in Paul’s day. The Restrainer has been continually and
effectively restraining for nearly 2,000 years so far. The Restrainer
is powerful enough to suppress the spiritual powers of darkness
seeking to promote the man of sin. The restraint is global. Numerous
suggestions have been made concerning the identity of the Restrainer:
Several of these views do not necessarily involve a supernatural
force. These include the Jewish state and James, Paul and the
preaching of the gospel, the Roman Empire, and human government. Other
views may be grouped as hostile supernatural views, which include
Satan, a hostile false prophet, a general hostile force in the form of
the mystery of lawlessness and human government, and the preincarnate
state of the man of lawlessness. In several views ὁ κατέχων [ho
katechōn] is seen as a benevolent supernatural figure rather than a
hostile one. Usually an angel, such as Michael, or another type of
heavenly being, such as Elijah, or a mythological being, is suggested.
The most common supernatural figure suggested, though, is God Himself.
I don't know of Dr. Pentecost's input into the NKJV, but Dr. Farstad's influence would clearly suggest a Dispensational reading of 2 Thess. 2:7. Dr. Pentecost's seminal work, "Things to Come", which is required reading for all Dallas Theological Seminary students, would be the dominant understanding of the text, therefore "He"(The Holy Spirit) would be understood as the Restrainer.
To suggest "he" as the Man of Lawlessness would be out of context; the "he" restrainer has to be removed before "that Wicked" in verse 8 can be revealed.
I believe the translation supports "he" over "He"-even the NKJV allows for it. But a staunch Dispensational view apparently prevailed in the NKJV.
