Welcome to the Ask an Expert forums! I'm intrigued by your son's assignment; hopefully we can help you come up with some ideas for building your model. One thing that would be helpful for us to know are any specific requirements that your sons teacher has set (e.g. the model must be less than 10" tall, the model must by built out of common household materials, the model must show the renal artery, etc.) Another point that it would be helpful to have clarification on is whether your son is to (a) build a model showing the anatomical parts of the human kidney or (b) make a model showing how the human kidney works. The way I would approach these two different projects would be quite different.
If we're talking about making an anatomical model of the human kidney, it makes sense to start with finding out what the human kidney actually looks like. This image is a fair diagram of what a cutaway of a human kidney looks like, and shows the names of each of the kidney's parts: http://en.wikipedia.org/wiki/Image:Kidney_PioM.png
. If you son's project is to make an anatomical model of the kidney, did his teacher provide him with a list of parts to include in the model (e.g. renal capsule, ureter, etc.) It would seem to me that a middle school student would not be expected to all 17 anatomical features shown in the diagram). If your son's teacher has not provided a list of anatomical features to include, I would suggest that you help your some pick key features to show in his model (consider including the features shown in this image: http://openlearn.open.ac.uk/file.php/28 ... 1_028i.jpg
or this one: http://www.freewebs.com/soaring_sphinct ... y_web2.jpg
) By looking at these three images, I think you should have a good sense of what the most important features are.
So, now that you and your son have decided which features your are going to include in your model, it's time to figure out exactly how we are going to build the model. The links barretttomlinson posted are quite informative, but I think that it's a bit beyond a middle school student's science project to build a working, artificial kidney.
I think that probably the first thing to recognize is that it will most likely be easiest to make your model as a cutaway, similar to the way the diagrams I provided links to are drawn as cutaways. Since the kidney is shaped like a kidney bean, you need some way to create a kidney-shaped outline. One idea that comes to mind is taking glued or stapled together strips of cardstock and then putting then inside a shoebox in such a way that a birds-eye view of the open box would show a circle (made of the joined strips of cardstock) inside of a rectangle (the shoebox). You could then pull part of the circle near on of the corners of the box inward and then fasten it in place, creating a kidney-like shape inside the box. (This is a bit difficult to describe using words without pictures).
You could then cut out pieces of colored craft foam (sometimes called Fun Foam) in the shape of the different anatomical features. (Perhaps you would cut out a red piece that fits snugly inside the cardstock outline for the renal cortex, a blue piece shaped to look like the renal vein, etc.) The cardstock would be the renal capsule. Do you get the idea of what I'm suggesting? If not, please let me know! The foam is very easy to cut, and you can stack layers of the foam and then glue them together (I've found that putting straight pins through the layers also holds them in place and has the advantage of being repositionable). So, for example, you might have your red renal cortex layer on the bottom of the box, a pink layer shaped like the renal medula next, then a tan piece shaped like the renal pelvis for the next layer, etc.
You can buy this at a craft store (like JoAnn's Fabrics or Michaels) for something like 75 cents a sheet. I know that the Walmart near where I live carries it, so you might try Walmart if you don't have a craft store nearby. If you ask for fun foam or craft foam, people will know what you are asking for and can point you in the right direction. It's usually found near sheets of felt, bottles of glitter, etc.
Let me know if this is helpful, if you need clarification, have questions, etc. If your son's project is to model how the kidney works, post back and I'll brainstorm ways to do that.