Some different bedding and accessories will make already a huge difference, whilst removing too childish items and toys. I browse at Ikea may inspire you.

Removable stickers are great – pretty easy to change as he ages, and not too expensive.

My son always liked picture of motor cross bikes, even his quilt cover had them on. Find out what interests him and go with that theme.

Posters of whatever they’re into, such as cars, space, robots etc.

Let them pick a paint Color, then I would recommend going on eBay and buying a wall decal with their name on it you can also get wall decals of trucks superheroes or anything your little one loves, the are all really inexpensive. But something as simple as a new doona cover and a rug/mat on the floor can really change the way a room looks.

I guess, what things is he interested in?
A paint job could easily make it look more boyish.
Is there a particular theme you want to stick to or just a bit of everything he likes?
Looking at apps like pininterest could help heaps. Just have to type boys bedroom makeover and i am sure it would bring up a heap of ideas.
I think the bonus is you can make some big changes at little prices.

Kmart has a good range of boys decorative items at reasonable prices.

Themed ideas are always easy to stick to and can be easily decorated throughout with posters, photos, figurines etc. My 15 month olds room is themed with Tinkerbell at the moment so we have a few dolls on display, lots of purples and greens, fairys, toadstools and grassy type carpet squares so its more like a specific woodland fairy theme. As they grow up you can start removing features and change it up a little.

I think posters are an easy way to decorate a child’s room with whatever they find interesting. My daughter loves Sherlock Holmes at the moment, so there are two posters displayed in her room. When her interests change, we’ll probably update the posters.