Add ceiling to floor drapes. Get rid of all mini blinds. Put a bench in his favorite sports team color under the flags(?).

I would put the race tickets all the way around the room at the top of the wall like a border around the ceiling. Might sound weired but Ive seen this done and looks pretty good. I would move the shelves to the wall over the nightstands. One to two as a backdrop for the lamp. looks too low and out of place where you have it. Do a gallery of pictures with all the pictures. They look lost being so small on a large wall.

Curtains need to go floor length.

For a unique headboard purchase exercise Matt and mount on the wall us can find these in a multitude of colors. You could place wooden frame around it if you like. Maybe evergreen color would provide nice contrast. I also would get some hockey sticks and hang over window as curtain rod then take those flags hang from hockey sticks with clips or ribbons in center place your curtains on either side of flags. You can then take your bicycle pic and hang on that wall as your matts will be enough for over headboard. I would add a comfy chair small metal table with fav sports team on it with lamp. Also replace shade on lamp in pic contrasts with bedding. Good luck

Thank you so much for your suggestions! Think I'd have room for a bench and a chair on the race ticket wall?