I've been looking into this as well for my DD12. The SAT math questions are not that difficult--meaning they are easier than math problems you'll see in those math competitions for kids. And the range of topics is quite limited compared to what you might expect. For example, there are no calculus problems.
Extrapolating from what you've said that he can already get 600 without studying or having had formal algebra study, I believe that targeted exposure to several topics in algebra 1 and 2, geometry, and trig would be likely to help his score go up over 700 within a year. If not, then certainly within 2 years.
But be very careful not to "teach to the SAT" because I found when I skipped through too much, it took the fun out of learning math, even though my DD could handle it. Now because she is older, I only teach her when she asks me to, and I make sure to give her a more well-rounded experience, and it works out fine because she gets her math "playtime" in before coming back to me to learn more.
Just for fun, I'll share with you our last topic that we discussed--how to multiply binomials. This came about because we were chatting about solving linear equations and coming to a lot of good conclusions, so I suggested we move on to thinking about how to solve a quadratic equation and compare the two types of equations to get more perspective.
I told her that in order to solve an arbitrary quadratic equation, she first needs to be able to factor trinomials of degree 2. She asked me how to do that and I said factoring is the opposite of multiplying, so first she should practice multiplying things that will give you trinomials of degree 2.
I remembered that her 4th grade teacher had taught them to multiply binomials (with numbers and no variables) and so I could connect it to that. She accepted it pretty readily that you could multiply 2 binomials and get something with 3 terms back, and then she seemed to lose interest so I stopped there, thinking that was plenty for one day.
Next time she asks me to teach her more about math, I will give her several different types of binomial multiplication problems and let her work through them and discover the patterns as she goes along.
Also, I take great care to read through the school's curriculum and try not to teach anything that will be taught in school, to prevent her from becoming extremely bored in school when the teacher starts teaching the same thing. Or if I do, I'll teach it in as different a way as I can think of. Very rarely have I had to teach her something that I knew would be taught in school.
