<p>More on physics and engineering, from what I have observed.</p>
<p>At the high school level I think physics, at least AP Physics with calculus, is the first time most high school students have encountered a science where a relatively high level of mathematics is essential. While biology and chemistry may get to a more mathematical representation of the subject area at the university level, its generally not so at the high school level. Figuring out how to model the phenomenon in question, choosing the right mathematical technique to represent the system, and then using that math to predict the behavior of that system under an arbitrary set of conditions, is the key to calculus based mechanics and much of engineering.</p>
<p>So it may be useful for your son to do some introspection and figure out why physics was so difficult. In my case regular high school physics was not easy but AP physics taken concurrently with AP calculus was fabulous. But as in many subjects being good at something in high school and enjoying it does not necessarily translate into being successful at the university level. After starting out as a physics major I switched to mathematics and ended up with an economics major and math minor. </p>
<p>In addition to differential and integral calculus of a single variable, multi variate calculus, linear algebra and differential equations are all going to be key for engineering (as they are for physics). For physics and intro engineering it takes both the mathematical horsepower to solve the math of the problem, but also the imagination/intuition/creativity or whatever to translate the physical problem into the correct mathematical framework.</p>