A.T. Robertson Commentary Acts 7:56

"and said, Behold, I see the heavens opened, and the Son of Man standing on the right hand of God." — Acts 7:56 (ASV)

Opened (διηνοιγμενους). Perfect passive predicate participle of διανοιγνυμ (cf. Matthew 3:16; Luke 3:21).

The son of man (τον υιον του ανθρωπου). Elsewhere in the N.T. in Christ's own words. Here Stephen may refer to the words of Jesus as preserved in Mt 26:64.