I see no issue with blue walls with that wallpaper but maybe not all the walls and maybe a different rug if you go a blue wall. IMO the wallper was your choice not his and as kids age they start having ideas about their space so maybe time to remove that wall paper and do something more him. I see nothing that says a kid is in this space . I like a twin bed with trundle for older kids for sleepovers and maybe get him to tell you what he would like on the walls and also let him choose the bedding. This is IMO his space so ask before doing much of anything.

Bedrooms are usually designed around the bed linens, so have him pick out a duvet or comforter before painting. Wayfair has a huge selection and you can also look at the standard mid range furniture retailers and The Company Store. I recommend you get him an extra long twin or queen sized bed if you expect him to end up being tall and you don't want to have to redecorate again before he leaves home. If he has an interest that he wants to incorporate such as a sports theme give that some thought. My son also requested blue walls and wall to wall carpeting, but I chose the shade of blue and the carpet. He also wanted a desk, but not all kids use them. There's no need to have one in your child's room if he won't use it. A queen sized bed allows the room to become a guest room easily when they leave home.

Lots of inspiration for teen boys' rooms on this site. Here are a couple of ideas for you to consider:

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Bring in lots of texture and accent colors. Use the wallpaper wall as the anchor for the headboard and think about bringing in painted nightstands in orange or even green.

I wouldn’t worry about room looking smaller and if it does, I’m sure your son would trade for a room he loves. I’d keep wallpaper if he likes and in a few years when he doesn’t, just remove then.

He probably hadn’t given much thought, other than color. Maybe show him some inspiration rooms to get a better sense of what he likes/needs. Would he prefer a trundle or a larger bed, desk, bookcase, shelving, a lounging chair, etc. Once that is figured out, decisions will be easier and room will probably have a longer lifespan. I’d paint the 3 walls navy and add a light rug abd bedding to offset.

We think you might be able to keep the wallpaper and just paint one other wall blue. Then perhaps look at getting a rug with less blue and some other accessories to add to the mix. See our two designs below for some recommendations on rugs, window treatments, and various other decor items.

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Some art you might like, Attached some pictures to give you better idea :) I hope it will help to decor your space :)

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art on paper · More Info