My DS took SAT twice in 2018:
7th grade: 1260
8th grade: 1330 (Math 10 pts+, Eng 60 pts+)
What is best way to improve his math score? How about Khan academy? Thank you.
The first way to improve the math score is simply by taking more advanced math at school. For many students, their score will naturally improve after finishing algebra 2. There is no rush; your child literally has another four years before he will need to reach his highest potential test score.
His math scores will improve when he takes the high school level math classes. He doesn’t have to take the SAT for at least 2 more years, I wouldn’t be too concerned for a 13yo right now. Have him continue to take a rigor course load when he is in HS and spend 8 - 10 weeks prepping before the next SAT (and I recommend taking it back to back, as most kids improve their score on the 2nd test). Lastly, in 10th/11th grade take some ACT practice tests to see if your son performs better on this test.
With a 1330, your son is in the 98th percentile of all eighth graders in terms of SAT performance. He is already doing amazingly well compared to his peers, and in another few years, I can guarantee you his score will go up by well over 100 points by virtue of nothing more than him having learned more math and becoming more experienced in grammar usage and essay-writing. (Many of the questions on the test require skills learned in Algebra 2, so the only way to improve his scores in that regard is to have him finish Algebra 2).
If you really want him to study for it - beside taking a prep course (which is a) expensive and b) an option that is not really available to middle schoolers), I’d recommend getting a Barron’s SAT prep book, making a list of all the math/grammar concepts he is unsure about, and studying each one individually using Khan Academy or another study aid. As for the reading section and the essay, practice tests are the main way to improve.
That being said: speaking as someone who took the SAT and ACT multiple times in 7th and 8th grade, I encourage you to not have him take the SAT again until about 10th grade, for two reasons. One, he most likely will not experience any massive score gains until he has completed the requisite courses, and you run the risk of him getting frustrated/discouraged. Two, assuming your son is looking at top schools, many colleges will not accept test scores that are more than two or three years old - I received a very good score on the ACT during eighth grade and was planning to submit it to colleges, but am now finding out that about half of the schools on my list are refusing to accept it.
Best of luck to you and your son 
The best thing you can do right now is don’t focus on test scores. He needs to explore other interests. Nobody is looking for professional standardized test takers. It makes for a boring resume.
Agree, there is no reason at all to focus on SAT scores for an 8th grader. He should follow the natural progression – take HS level math classes, prepare for the SAT when the time is appropriate (junior year), and take the exam again at that time. IMO there is zero value in taking the SAT early (unless it is needed for a summer/enrichment program). For now, your S’s time can be much better spend on other things.
consistent practice was key for me