Your little boy sounds so sweet! I know I’m not the Mr. Kate you’re looking for, but your situation sounds like something I can try to help with. 😊
If I can, I’d like to suggest giving him something that is interchangeable, so when he gets older (and he will), he won’t be embarrassed to call that room his own. Many minimalistic designs have the potential to be decorated to a certain extent, but for a 5-year-old boy, it might not be the dream. However, as he gets older, I’m sure he will enjoy having a “cool” room, so I wouldn’t give up on your theme.
To keep the minimalist vibe going, I would suggest a low-rise bed frame (maybe black and white vertical stripes or grey) with white sheets and a yellow throw blanket. As a little kid, anybody loves lots of pillows (and honestly, I still do), so I’d suggest some yellow pillows and some white ones (maybe with some yellow or gray airplane or car designs).
If you have tiled or wooden floors, I’d say a comfy, fluffy rug is a must-have because a little boy is always going to be playing on the floor. Lots of ottomans are good to store toys in a minimalist fashion, while still keeping them at easy access.
Now, your big problem: getting your little boy’s rainbow My Little Pony and aircraft/model car passion into that room without too much clutter. As you know, the most common minimalist form of expression is photo frames. I think it would be really cute to hang up individual pictures of My Little Pony characters and airplanes/cars in varying frame sizes, sorted by color. You could also have yellow bedside lamps with black or grey airplane patterns on it, which wouldn’t be too much, but still have that element of childish play.
I hope I helped you with your room decor! Good luck decorating for your little boy!