Levitate a frog.

Make and demonstrate Thermite

Build a tesla coil
...or a Van de Graaff Generator.

A lot depends on what you have access to or can gain access to. Thermite is always exciting but finding some of the components can be tough unless you are willing to purchase from a chem supply company.
Nitrogen tri iodide will produce a good bang but you will have to prep it on site and it cannot be disturbed while it dries...and especially when it is dry. If you son can be given access to a fume hood in a lab that will not be used for a few hours, that is a possibility.

Check out Flinn Scientifics website. There should be some good chemistry demos on there that may work. They used to include theory with their demos...not sure if they still do but it is worth a look.

We buy all our science equipment from this website. They have some ideas for projects.

Science Projects for Kids | Chemistry, Life Science, Physics

How about a subtle automotive one: Determining Friction Co-efficiency of Common Household Lubricants.

-Have a motor or two(hi/lo) driving a used crank through gears(pinion/square cut).
-Crank is positioned at the top of large flat plate on an angle.
-Attached to the crank offset bearings are cables attached to solid objects, which are dragged up and then slide down.
In the middle of these cables is some kind of pull-scale(such as for fishing).
-The path of each solid object is lubricated by a different product.

All factors are interchangeable and create mass data for bulk interpolation: Position, plate angle, temp, speed, mass of object, contact area, etc.

My son won first prize for an experiment describing plate tectonics. You take a cement block attach an eyebolt to it. Take a long bungee and thread it into a ratchet tie down crank which is bolted to the boards(6' long).
The block is placed on a board with gradations to see how far it moves. There is a scale to measure the stored energy before the force to move the block is overcome by the friction.
Very easy to make, simple design and demonstrates the built up energy very well.

G50-
Your kid has a tough HS teacher? Consider yourself fortunate to have such a teacher today!

Anyhow for a science fair project, here's what I did way back in 8th grade and with bragging rights, advanced to the state level competition at the university and won my class. Coming out of my only year at a small parochial school up against big public schools wasn't too bad either.

Have your kid build an electric motor out of household stuff (do not buy a kit - not impressive). Have them explain 'how does an electric motor work?' Give examples where used, types of and the future. So onto making one: I made a DC motor out of a wood base, wood dowel - metal rod (for the armature), metal rod bent for the fields, wire, tin snipped from pop can for the brushes and two ordinary bookshelf -L- type brackets, misc. nails. The working motor was approximately 20 in. long by 12 in. high. Not difficult, interesting and lots of show!

When in high school, I made a very simple working carburetor and showed atomization. I explained the purpose and how they work but nobody seemed to care or understand... laugh.

Whatever you decide, its great that your involved with your kid and looking for suggestions. Enjoy

First figure out if it's a "build and characterize cool stuff" or a "rigorously follow the scientific method" contest. There's a difference between the two.

If it's a scientific-method-focused one you'll get a leg up by simply following directions. Our county's science fair is like that. We (my professional organization) judge it every year - serious deductions for not following the scientific method AND for having obvious parental involvement.

Oh - don't do plants or other bio stuff. Everyone does that...

Map the RF field of a Tesla Coil.

Yes, that's spot on.

It's a "rigorously follow the scientific method" type thing. So, need a hypothesis, test procedure, control, etc.

Teacher has already warned about plant or other bio stuff! Too common.

I'm pretty surprised at the level of work that is expected of them at his school, and the volume of work, too.

<iframe width="420" height="315" src="http://www.youtube.com/embed/qxuGDEz8wDg" frameborder="0" allowfullscreen></iframe>

Now that's funny :D

What's he into? Earth Science, Physics, Reproduction, etc..?

He likes pretty much all aspects of science.

Something re physics would probably be cool. Something with physics, auto related? He likes cars!

He's also a golfer, that's his real passion, so we were brainstorming some ideas for a golf physics experiment. It's tough because repeatability of a golf swing is difficult, as is most measurements/data recording.

Golf ball dimples... lots of science there!

You could set up some sort of air gun to launch golf balls with very little variation between shots.

build wind tunnel. show difference in laminar airflow between smooth ball and dimpled ball.

Reproduce the experiment in which an electron simultaneously exists in two places at once!

(seriously - it's been done).

Or, through an elaborate flow-chart of energy, show that the Prius, Volt, Insight, etc. is less efficient than taking a steam train to work in which you heat the seats with a woodstove.

As for golf. How about what makes the 'ping' sound in a good hit as opposed to the 'thump' of a bad one.

Take care!