My son did that when he was two. He specifically wanted to be a green crayon. We got a pair of green sweatpants and a matching green turtleneck, did up a fair imitation of a Crayola® label on the computer, printed it out on iron-on transfer material, and ironed it onto the shirt. The piece de resistance was the green plastic flowerpot we got for him to wear, inverted, on his head (for the point of the crayon). Went over very well, wasn’t at all restrictive or unfortable for him to wear, and was fairly easy to do. The hardest thing was getting the flowerpot to stay on his head. We ended up putting it over a khaki bucket hat – given more time I’m sure we’d have come up with a better solution. You can see the results here.
My cousin did this when we were kids, and won the local costume contest. Those old Fruit of the Loom commercials inspired him.
Saw someone at a party once who dressed up as a bidet: he had a toilet seat around his neck and was carrying a squirt gun. Probably not a kid’s costume, but it got a lot of puzzled looks (and laughs when people got the joke).
My mom used to hand make all my costumes and they were so much better than the ones you buy at the store that are just a plastic “suit” and a mask. Plus, I think she always felt the store costumes were too expensive and she could do something much better for the money.
Most costumes she made had a leotard base that she added stuff to. I was a bumble bee one year, with black leotard & tights and that yellow furry stuff sewed on the torso in stripes. Had a black eye-mask and for antennae, a headband with coat hanger wire attached to it, covered in black material and little yellow bows tied on top.
Another year, I was a devil, with red leotard & tights, red eye mask and she sewed a tail (coat hanger again, covered in red and stuffed with cotton) and horns (same thing for the horns, only sewed onto a read headband). Had a red sweater on for going out at night.
My absolute favorite was the year she made a fairy princess costume. Pink leotard & tights, to which she sewed pink tulle (sort of like a tu-tu) and silver sequins. Then she made a tiara out of sparkly pipe cleaner and these pink jewels, and my grandpa made a magic wand by cutting a star out of plywood and spraypainting it & a wooden dowell silver. Then he glued some of the pink jewels on it. The only bad thing about that year was that it was freezing out and my mom forced me to wear a blue coat over my fabulous pink outfit. I would rather have gotten frostbite, but she made me anyway. 
The best part is, I still have all the stuff she ever made for me. I look at it now and wonder if a kid today would love and appreciate those costumes as much as I did. I hate to imagine the day I bust out the old bumble bee costume for my kid and she bursts into tears and begs for the plastic store costume!
Anyway…those probably aren’t the best suggestions for a boy. As a kid, the boy next door pretty much was a box every year. I mean, it was variations on the box, but basically his costume each year consisted of a box made up like something box shaped. Dice, computer, christmas present, television, etc. I always thought it was pretty boring, but he loved it. One year he did take an old bean bag chair, painted it like a soccer ball and cut holes in it for various extremeties. It worked ok for a little while but all the filling came out the leg holes. That one needed some refinement.
I think a vampire is good for a boy, and easy. Black clothes, white face, fangs and a cape. Plus, capes are really easy to make if you don’t want to buy one. How about a coal miner? Dirty clothes, dirty face, and a hard hat with one of those head-mounted bike lights.
When I was a kid, we kept a box of old clothes around for use as halloween costumes. Much of that old clothing had belonged to one of my 7 sisters at one time or another. My favorite costume was created out of an old pair of black leather pants, a yellow and black vertical-striped shirt, a bent coathanger, some cardboard, and a bunch of aluminum foil. I went as Captain Hook when I was ten, and again at 11 becasue we moved cross country during the summer. One of my sisters covered the bent coathanger with aluminum foil to make the hook and made a sword with cardboard and more aluminum foil.
A “missing child” - make up a box to look like a milk carton with the kid’s face peeking out the side.
Blue sweatshirt with cotton balls glued on it & a squirt gun – “partly cloudy with a chance of rain”
Linus Van Pelt. Jeans, black-and-red striped t-shirt, a length of blue flannel, and thumb in mouth. “Oh Great Pumpkin, where are you?”
I’m still hoping I can talk Mr. Rilch and Friend Across Town to be Charlie Brown and Linus, respectively, to my Lucy. Not much chance, though. Mr. Rilch doesn’t want to be a ‘loser’, and Friend Across Town (was going to acronymize that, but look at the letters and you’ll see why not) doesn’t want to suck his thumb all night.