‘What is man?’ He is speaking of the whole human race. The quotation comes from Psalm 8:4, where David looks up into the night sky and reflects upon man’s insignificance in the vastness of the created universe.
So now, very obviously Psalm 8, as quoted here in Hebrews 2 verse 6, begins to speak prophetically of Christ. ‘What is man?’ What a privilege: that alienated, sinful, disobedient mankind has been visited by God, and remembered in mercy by God! What an amazing privilege!