A simple plastic box (wider than it is deep) with plenty of ventilation, and drainage holes is the easiest start. Have you looked at redwormcomposting.com? It has some great videos about getting started.
Another option, but needing a little more DIY skills, is the OSCR junior bin. Run a google search and you'll get plans for it.

Easy Start:
1. Place a piece of cardboard on the ground. Wait 1 day. Look under cardboard and gather worms.
2. Put worms in opaque ventilated container. Repeat #1.
3. Add wet ripped newspaper and dirt and any organic scraps. Repeat #1.
4. Worms need moisture, air, proper temperature (60 to 75 approx.), and food. Repeat #1.
Worms are easy. Have Fun!

Hey what a great program I don't remember that from when I was in scouts

Worms are so very very easy dispite that some people make it sound hard I sometimes don't bother with mine for 2 weeks at a time

A bin should be less than $10 you probly have one laying around it can be a rubbermaid with holes in it thats what I use big hole in the bottom with a piece of screen or cloth over it slanted that way incase it needs to drain more hole in the side for air rip up some newspaper and cardboard and start thowing in scraps

Or nail together scrap lumber to make a box, use an old sink ect ect the possibilitys are endless

I do think a larger bin is better as they are not so sensitive an extra handfull of food won't disrupt your whole system the worms will have plenty of room to escape bad conditons

You'll need to get it going quickly if you only have 90 days - suggest you start the worms with some precomposted material as food - and use the blender to masticate vegetable scraps at first, to make it easy for your worms to get a running start. Suggest your boy keeps a diary, to note food given, any pests etc. What type of worms are you going to use?

Here is a link that might be useful: How to Make Your Own Worm Farm

He just has to build and maintain it for 90 days, there's no deadline. He has already grown and planted 3 veggies and flowers from seed and planted 3 more of each from seedling and doing a bang-up job keeping them watered and happy. We should be seeing squash and tomatoes soon, and the strawberry is threatening to produce as well. Not bad for my 10 year old computer geek and his brown-thumbed mama :)

Anyway, my thought was why stop at 90 days? Waste of good worms, i say! So I think we'll go the rubbermaid route, which I'll get this weekend, and have already started saving scraps. We'll get it started, let it sit for a week or so, and then I'll shell out for a pile of red wigglers. Once his requirement is met, the bin will really be more for growing more worms to feed to our hungry little box turtle - so we get the best of all worlds - less stinky trash, free turtle food and a fat turtle!

The requirement is either to compost or vermipost, and vermipost sounded like more fun to both of us :) Thanks for the tips, and we'll keep you posted on how it turns out!

If you want to get things jump started, feed them horse manure. Kitchen scraps take a few days to break down enough for the worms to eat. Worms go after horse manure almost immediately.

If you want your worms to really get breeding fast you could try composting aswell as vermicomposting. Feed the partially composted food to the worms- they love it. (And your son might get extra credit for doing both!!)

If you don't want to invest much money in the project, try using one or two plastic containers that you already have around the house. I am using two kitty litter buckets (one holding the worms with holes drilled in top and bottom, one to catch any drainage). I also bought only a small quantity of worms from my local pet store. It is a small set up but it has been fun and easy to maintain - and I've only invested less than $10 total.

In following up with idratherbegardening, I have had great success with 5 gallon buckets from Home Depot. I simply drill a ton of holes in the bottom (except for the the bottom bucket)of as many layers as I want. $3 each
They also have lids for $1 or so.

You might be able to get 5 gallon buckets from a local fast food joint. I have obtained somewhere around 30 or so over time from my local Burger King. They get sliced pickles in them and will have lots and lots over time.

Then you can add recycling buckets to your project.

Well, we're on our way! Over the weekend I purchased 2 rubbermaid totes and a giant trash can. My ever-patient sweetheart drilled millions of holes for me, and now I have a garbage can compost bin already starting to work (it has some cardboard and fruit/veggie remnants from cleaning out the fridge). For the worms we have one bin with no holes for drainage and a 2nd bin with holes for the wormies, I've put some cardboard in there for now. The plan is to get things started disintigrating in the compost bin, order worms at the beginning of next week, and feed them partial compost. While we were at it we got a 2nd trash can for recycling plastics. Koby learned a lot yesterday about what trash to put in which bin, and had a great time tearing up cardboard and dumping his apple cores in the compost bin.

This is going to be a great educational project for both of us :)

Thats great you would be amazed how much more thoughtful of an adult he will be getting exposed to things like this I hope everything goes super on his project