I believe He is the restrainer indwelling the true believers and will be removed at the rapture.
To begin with, 2Thess. 2:5 tells us that the Thessalonians knew what it was. Paul had told them when he was with them, and he was reminding them of this very thing.
Continuing on with 2 Thes. we know that the hinderer of lawlessness:
Is something already known.
It NOW hinders lawlessness.
It is strong enough to prevent the revelation of the Antichrist.
It will hinder lawlessness until it is removed from the earth.
It is called "he".
It will be removed from the earth before the second coming of Christ.
It will not be here when Christ comes to destroy Antichrist.
So, the "he" must be one of two things. Either the church or the Holy Spirit.
The Holy Spirit will be here all through the tribulation and forever, according to: (Jn.14:16; Acts 2:16-21; Rev. 12:17 with 19:10; Zech. 12:10-13:1).
Multitudes will be saved by the Holy Spirit during the tribulation, as now. (Acts2:16-21; Rev. 6:9-11; 7:1-21; 12:5,17; 15:2-4; 20:4-6).
If "he" is not the Holy Spirit, the only thing "he" can be referring to is the Church the BOdy of Christ.
And this makes perfect sense if you believe we are going to be raptured before the Tribulation begins. (Thessalonians 4:16). We know Paul was writing them to remind them of what he had told them earlier. That he was seeking to dispel rumors that were untrue, that he told them to comfort one another with his words, and that the Church would be kept from the wrath of God. All rapture information, imho.
One more thing, because I know it will come up: calling the Church "he". It is called the "one new man", masculine. It is called the Body of Christ, masculine. Read Eph. 2:15; 4:13, 1 Cor. 12:12-13, 27; Eph. 1:22-23; Col. 1:18,24.