J.B.
Hi - when I talk to parents about this (I teach it) we recommend starting no sooner than the summer after Sophomore year. Next summer would be a great time for him to take a course in preparation for taking the October 2014 exam. After he would be able to review his materials and the strategies he learned on his own to improve his scores even further before taking it again in the spring of 2015 (most big companies have extra practice material on-line that students have access to for self-study after the course ends). March and May are the more popular exam dates for the spring. Unless a student has a winter sport or activity that make March not ideal, I usually recommend March so that there is room to test again later in the spring if desired. June of Junior year is a great time to take subject tests so I usually recommend that students NOT plan on taking the SAT in June of Junior year so that they can get subject tests in.
If he ends up not doing it over the summer, taking a course in the fall or spring of Junior year is very common and is fine. The best time depends on how busy your son will be in either season.
The only testing to consider as a Sophomore would be subject tests in June if he is planning on taking any and is finishing up a non-cumulative course this year that he does very well in. Taking honors or AP Biology and has an A? Take the Biology subject test this year. Math and English? Wait until the end of Junior year.
It's great that you're thinking ahead - right now, his focus should be on his GPA (it counts for 50% of what colleges look at) and on establishing some depth in his extra-curriculars. Test prep next summer or Junior year, 2-3 SAT tests Junior year, subject tests Junior year, then the summer after that he can focus on his essay(s), visit schools, and get organized for submitting applications the fall of Senior year.