Here is the picture of his window. The room is enormous, but only 1 small window. However, he has a LOT of stuff in there too.

I would return the skimpy navy draperies and buy another yard of navy and make a boxed cornice. Then find some space themed fabric and make some draperies for that window that are not skimpy.
You say he wants a space theme....and so far I see none...nor any pattern. So for a few dollars of fabric he could have his space theme room and still have a basic blue comforter and striped headboard.

I think it looks good with just the blinds - no curtains.

Sorry if my post was confusing - he does NOT want a space themed room. His old room was space theme, and I kept the green/blue walls.

We don't want any themes now. He wants curtains, the white blinds don't block enough sun. Comment was don't see any pattern - what pattern should I add with the navy stripe and checked toybox and striped walls? I am very open to suggestions.

How about something like this?

{{!gwi}}

Horizontal stripes like the headboard?

How about Pottery Barn Kids navy blackout curtains? They are lined and heavy and I think they would look more generous on the window than the JCP ones you bought. They also have some new patterns out that your son might like. There is a navy pattern with thin pinstripes. There is also a navy gingham but your son might find that too babyish. Country Curtains has a navy check curtain as well but I would make sure they are lined so they aren't too skimpy.

FYI, same problem here and I bought draperies and there was still too much morning light. I needed lined panels. Like your idea of navy panels with with white. Penney's in my go to for draperies these days.