"The seventh angel blew his trumpet, and there were loud voices in heaven saying: The kingdom of the world has become the kingdom of our Lord and of His Messiah, and He will reign forever and ever!" (Holman Christian Standard Bible)
The "He" who will reign forever and ever is God the Father. He is the "Lord God Almighty" mentioned above. Compare Revelation 1:4-8; 11:6-17.
"The radical theocentrism of John's revelation is not heard more clearly than here. The antecedent of he will reign for ever and ever is God rather than either Christ or both God and Christ" (Robert W. Wall, Revelation, page 153).