Response to Photons?!
You do have a very smart son. He has caught an apparent contradiction that completely escaped me. The best I can do in way of explanation is to point out that photons are an entirely different sort of “particle”, so different that they should not be considered particles at all. It is better to think of them as fundamental units or just “dots” of light energy.
The key difference is that all particles of matter have mass (a certain weight in the presence of gravity). The more particles of matter that are packed together, the greater the weight. Photons have no mass. Regardless of how many photons (how much light) you shine into a bucket, there is still no weight, and there is no way that you can pack photons together and get a mass with size and shape.
Another and very significant difference between photons and particles with mass is that light (photons) can be easily seen as waves, not particles. A simple activity described in the text explains how to do this. Even physicists have no way to explaining why/how photons behave as both waves and particles, but it definitely sets them apart from atoms and larger particles of matter.
In conclusion, photons can not be considered as particles in the same sense as particles of matter.
The second part of your post is also significant. Please reread “Students’ Questions and Suggested Responses, Type 5 Questions” (page 19, Vol. I, 2nd ed.)
This may well raise further questions. Please ask them.