Assuming he's agreed to do this, you first need to pick out what kind of princess he wants to be. A generic "pretty princess?" A specific real human princess? A Disney princess?
Once you've worked that out, figure out what outfit he'll be wearing and figure out where to get it. Most of the time, a "princess dress" is a long gown, usually with a full skirt and often some kind of crenoline/petticoat to flare it out. For "generic" princess, thrift stores can be a good resource. For Disney, there are tons of official outlets and private sellers like Etsy crafters you can buy from. Think about how much you want to spend, and also how "realistic" you want the outfit to be.
When most people think princesses, they think about heels, so pick out some shoes that match his dress. Buy or borrow. Try not to go too high unless he really wants that look. He's probably not used to wearing heels. Also, boys tend to be taller than girls, so a boy in sky high heels can look unnaturally tall. Make sure you get his dress hemmed wearing the heels for his costume - it's important to get the length right so he doesn't trip. Practice walking in the heels.
For makeup, if he's wearing a gown, you can go pretty glamorous. Full red lips, good mascara (or false lashes), a decent amount of eye makeup, possibly glitter. The works. Do his nails (fingers and toes) to match.
Long luxurious hair is generally what you're looking for. Assuming his hair isn't that long, invest in a decent wig, and make sure it fits. You can go a lot of ways with styling, but generally something like you'd wear to a wedding or a formal dance at a minimum. You can go over the top with elborately braided or layers styles.
Jewelry is a must for a princess. If his ears aren't pierced, and he doesn't want to go there for a costume, get some good clip-ons. Something that sparkles and dangles. A nice necklace, a bracelet, rings. It's a costume, and this is just the place for costume jewelry. It doesn't have to be real, or expensive, but it has to look impressive and make a statement. He's a princess and should look the part.
My first time officially "as a princess" was for halloween. I (a redheaded boy) was Princess Anna from Frozen, and my girlfriend (now fiancee, a blonde girl) was Elsa. I'd been dressed as a girl before and enjoyed it, but never so formal and never in public, so we talked about it a lot before I made the decision to go for it. She found an amazing crafter who built both our dresses to measure (I spent a fair amount of time in a bra and corset getting measured for it). One nice thing about being Anna was that I could wear boots (though I did wear heeled ones). I have fairly long hair for a boy (it was shoulder length at the time - longer now), and rather than a wig I got extensions so I could have Anna's trademark long braids. It felt amazing to get all dolled up in my fabulous dress and makeup. I was more than a little nervous going to class and later to a party looking so girly, but it was Halloween, so I could call it "just a costume."
Put him in a pink dress tights/legins and make up
Put him in a princess dress and a wig
Any dress a princess wears would be a princess dress.
No, Princess Daisy does not wear a pink dress, Princess Daisy wears a yellow dress with a flower in the middle, Princess Peach wears a Pink Dress.
A fancy dress for a princess could be a gown.
Twilight Princess
It makes your boy turn into a fairy freak who will dress up like a princess.
The ISBN of The Boy in the Dress is 0007279035.
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You look like a princess in your new dress.
I'm a boy and love dressing up you silly fools and bubble gum Netherlands 2-1 Brazil!Let's dress Melo in princess comstume hurray! Miss QQ1
The Boy in the Dress was created on 2008-10-01.
There are several games similar to Cinderella Dress Up. Some examples include "Princess Makeover", "Waterfall Princess Dress Up" and "Cinderella Ballroom Dress Up".
Princess Anne was there wearing a purple dress and floral coat