Help me paint my son's odd room!

2 years ago
My son is turning 9 and I need help with his room. When we first moved in he lived with what was already there, but last year we redid my daughter's room and now it's my son's turn. He originally picked a bold, bright yellow. I painted 2 walls with the color he picked and realized it was just. too. much. It gave me a headache just to be in there and the yellow glowed into the hallway! I primed over it and we tried a few (OK, a lot of) other yellow paint samples. There was nothing that worked.
Hoping to persuade him away from yellow, I let him pick out new bedding and I love his choice. However, he now wants to add in the colors from his bedding to the yellow walls.

His room is long and narrow (15x9), has the angled walls and the ceiling is low (only 7.5ft) and it has a deep dormer. We plan to keep a twin bed in the dormer because my daughter's room is a guest room a few times a year, so having an extra bed there works. (We have matching bedding for the twin bed)

So, how best to paint the room? All one color? Accent wall? If I do add stripes in the navy, green and bright blue will it chop up the room too much?