<p>Your son should go for the acceleration. If he can learn the
earth science by reading the book, ask to keep the book, and
have him read it. If he could informally take the final after he
reads the book, I’ll bet it would give him a lot of confidence
to prove to himself that he can learn on his own by reading.</p>
<p>Earth science is great (I have a degree in it myself), but <em>real</em>
earth science requires quite a background in all the hard sciences.
So, no harm done in going ahead with the high school sequence
even if he should decide in the end that he wants to seriously
study earth sciences.</p>