A few thoughts:
-One benefit to taking Dec SAT is that the material is fresher, some downsides are that immediately on the heels of that (maybe concurrent) is first semester finals, and then have to start preparing for Feb ACT
-Has he taken a full, practice ACT? If so, how does that score compare to his SAT score? Some kids definitely have a tendency to do better on one test rather than the other
-Did he take an SAT test prep class or have a tutor? If not, perhaps add that for Dec SAT and/or Feb ACT
-If merit hunt is priority, your S may have to take more tests than everyone would like
-Much will also depend on your S’s tolerance to continue to grind junior year—the demands are intense when one considers standardized testing (SAT/ACT/subject tests/AP tests), school work, ECs, many have junior research papers, etc.
-I would not send the free scores upon registration. So many schools added self-reporting of test scores as an option this year, and by next year’s cycle many more will probably allow it as well, meaning you might not have to pay to send many. While I believe schools when they say they will only consider the scores that put the app in the best light, why send low(er) ones? Unless it’s the few schools who want all test scores