He can't be talking about the fourth one as if it's a son of pagan gods. He knows that the three worship only one God and he has to recognize the fact that the angel was sent by the one God because what other God would help them? :
Dan 3:28 Nebuchadnezzar spoke and said, Blessed be the God of Shadrach, Meshach, and Abednego, who has sent His Angel and has delivered His servants who trusted in Him, and have changed the king's words and have given their bodies that they might not serve nor worship any god except their own God.
This was his first impression of what he saw. After the three were released from the fire, the king concluded that they had been saved by their God, who had send His angel to protect them.