Many students have a harder time with the no calculator math than the calculator math. Once reason might be that it’s a shorter section and therefore more time-sensitive section. How does your son do with time on the section?
A few pieces of advice that might help …
In addition to re-doing his incorrect problems, and re-writing them in pen, I’d suggest he re-write the entire problem – question and solution – into a looseleaf notebook, one problem per page. He can later arrange the pages by topic (put all circle equations together, for example), and then review the notebook regularly. This will help him really absorb and master the problems he had difficulty with.
If timing is a continuing issue, he might consider doing problems 16-20 first. These are the grid-ins. The reason I sometimes like students to do these first is that if they end up running out of time, it would be impossible to guess on questions 16-20, but if problems 11-16 were not yet addressed, he could guess (multiple choice) and have a 25% chance of getting each one correct. This technique might end up saving 1-2 points, possibly.
He should bubble the answers after every 2 pages of problems (i.e., bubble right before he turns the page). This will save time versus the approach of bubbling after every answer, plus it will increase accuracy of bubbling. Of course, during the last 5 minutes or so, he should bubble after every question.
Make sure your son looks carefully at WHY he gets each of the incorrect answers wrong. If it’s because he doesn’t know how to do the relevant math, he needs to learn the math (obviously). However, odds are there are OTHER reasons why he’s missing them. Did he answer the right question? Believe it or not, this is the cause of many errors. The SAT might ask “what is x + 10” and students focus so much on solving for x that once they do, they just select that answer (say, x=6). The SAT knows this and will always have “6” as one of the answer choices. Every student should re-read the question asked right before choosing the answer. Other common errors are to mess up units or confuse “less than” and “more than” etc. It’s important that your son look at ALL the reasons he is making errors and treat each error as serious as the error from not knowing how to do the math. He should also consider whether he is writing out the solutions in his book. Many students are so confident in their ability to do work in their head that they don’t write it out, thinking that they are saving time. They are not. It’s impossible (or extremely hard) to double-check work if you haven’t written it out. I tell students to show their work well before they think they need to.
Don’t know if any of this will help, but good luck!