If main pressing goal right now is to prepare for the SAT essay section I would not waste time with general writing books or courses. The SAT and ACT essays are really a different kind of writing than what he needs to do for other purposes. The essay is absolutely something that he can improve his score on by knowing the strategies. So, I'd focus specifically on resources designed for this purpose and treat this as the game it is.
Start with the College Board site for an explanation of the essay portion and strategies
http://www.collegeboard.com/student/testing/sat/prep_one/essay/pracStart.html Look at a New York Times article about how essays are graded.
http://www.nytimes.com/2005/05/04/education/04education.html he might enjoy this too http://www.latimes.com/news/opinion/sunday/commentary/la-op-sat3apr03,1,3742834.story
Either buy an SAT prep book or get one from the library. You'll find plenty with sample essays he can look at. I wouldn't worry about practicing editing right now as that is the opposite of what he needs for these tests. For the essay he needs to practice writing fast and not worrying about polishing. Scroll down here to see some sample prompts for the SAT
http://www.onlinemathlearning.com/sat-test-prep.html I suggest he try to write a few timed essays.
The main thing I would share with your son is that these essays are graded very quickly. He wants to write a longer essay and use some good SAT type vocabulary especially in the first paragraph. Start with putting the prompt into his own words. Take a very clear position and don't worry if it isn't what he believes in his heart. Support his position with examples (he can prepare general examples ahead of time that will fit for a variety of prompts - a couple of famous people who have shown courage, a couple of important events that illustrate historical lessons, a couple of scientists, a couple of classic novels, etc.)For the SAT it doesn't matter if his examples are even true, it is all about getting something down on paper that sounds good - again this is very different than the kind of writing he wants to do for other purposes.
Oh and you asked how much it counts. The essay portion on the SAT is worth 1/3 of the total writing score. Many schools care a lot less about the writing score than they do about the math and verbal sections. Because some prep may really improve his scores I wouldn't ignore it but I would keep it in perspective.
