Nice. What volume/pressure is your accumulator going to be, what kinda projectile will you be using, and what's your expected muzzle velocity?
I completely disagree - I tutored plenty of people in math/physics in high school, and I can't count the number of times that I've had someone ask me, "what will I ever use this for in my adult life?"... And it's no surprise I got asked that, physics is normally boring as shit for most people - you run numbers through a bunch of different equations to eventually find out that newton got banged on the head by an apple at a velocity of 3.4m/s. Yawn.
The world needs tools like this one to make the sciences actually interesting and fun to do - making students absorb what they learn better, and perhaps sparking interest in a kid who up to that point didn't know what they wanted to do. A good teaching tool is definitely more important than some dumb (but omg, patentable!) gadget that gets cat hair off your clothes or something.
Once this gun is built, set up a challenge with it - set up a target some distance away from the cannon which kids have to hit. Give them the specs of the gun - air chamber volume, barrel length/diameter, projectile mass, etc - they have to pick the angle of the gun and the air pressure setting to hit the target. Offer prizes for hitting the target, and a grand prize for closest. You'll have people forming groups trying to get the equations figured out, people writing calculator programs or spreadsheets to go through them, etc. People will have a good time, and hey, they'll learn something. Which is the whole point!
And hey, maybe he's going to college to get an education degree *shrug*