Physics book for 15 year old boy
"Physics for the Inquiring Mind: The Methods, Nature, and Philosophy of Physical Science" by Eric M. Rogers is a superb book for a teenager. It doesn't overwhelm the reader with esoteric mathematics and provides a very solid foundation which is often missing in so many books. Expensive at about 60 dollars , I found a copy for 15 dollars, so seek young man and ye shall find!
On Amazon: Physics for the Inquiring Mind
Similarities in Physics - John Northrup Shive and Robert Weber is another great book for showing the interconnectedness in physics and basic ideas ranging from Simple Harmonic Motion, Heat, Noise etc. Actually I think this is one of the great and unknown pedagogical books for physics and it's free here. you really can't go wrong reading that one.
If you like a laugh and the kooky then try Macschrödinger's Cat - Jim O'Brien also at Amazon, but don't read this one for insight or learning, read this one just to stretch your mind.
Even if it doesn't answer your question directly i think these videolectures are a great place to start and really motivating (and for free):
Walter Lewin ot the MIT: http://web.mit.edu/physics/people/faculty/lewin_walter.html go to --> take a class Physics I, II, III
Foundations of Modern Physics: Leonard Susskind http://academicearth.org/courses/foundations-of-modern-physics