Just tell him what you told me: that there’s no character development, although you could maybe leave the part about the Justice League movie out. Character development is hugely important in any piece of writing, and if that writing piece doesn’t have it, it’s not going anywhere.
He may be a bit pretentious, but trust me, if there’s a major flaw in his work he’ll want (and need) to know about it.
And yes, as one of my followers mentioned, tell him gently, because he’s just trying his best.