Can you change the sensitivity of an EV3 Touch Sensor?
It is not possible to increase the sensitivity of the touch sensor in-programming. Generally, you can use a lever to decrease the amount of force required.
 

Here the "load" is the touch sensor button, the "effort" is the ball hitting the lever, and the beam is a lego beam.  

If the ball you're using is too light, you still probably won't be able to depress the touch sensor, regardless of how long the lever is. You can experiment with a heavier ball and a lever arm to try to get it working.  

If you can't get a mechanical solution working, or if it is required that the ball is only detected when it hits a small target, a light/color sensor might be the next thing to try.  

Side note, I think it's really great you're helping your son with his project. Feel free to post back here if you have more questions!