First, this is unusual, I think most kids do better the second time around than the first as they can concentrate on studying the areas they missed the 1st time and have experienced the real, timed SAT and know what to expect.
Second, I would take the second test at the end of the summer 2020 but get a private weekly tutor that will work with your kid who gives “home work”, does practice tests, shares wisdom on test strategy, etc. to keep him motivated and on track so YOU don’t have to do anything. It’s money well spent in my opinion, especially if you have a kid who isn’t a self-starter to prep well. Treat it like a summer school class in July and August (6-8 weeks).