There's definitely something off about ol' Julian, but is he the Son of the Morning?
I'm not convinced the leader of New Delphi is the fallen angel we're expecting, despite the black eyes at the end. He certainly didn't behave like an older brother or the most powerful of the archangels in the flashbacks. Gabriel referred to him as a higher angel.
That said, it's wonderful that Dominion Season 2 Episode 3 raised so many questions. Julian may not be Lucifer, but he is one of the fallen, and there seems to be a connection there.