If your son is having trouble studying for the SAT for more than an hour a day, then I would recommend starting by studying individual sections, then working up to a practice test.
If he hasn't done a full-length practice test already, of course, he should do this to get a baseline of where his score is now and to figure out what he has to improve. Once he's done this, however, his time would probably be better spent focusing on individual sections and thoroughly reviewing his mistakes. Since he can only focus for an hour at a time right now, he'll at least be able to get into some good targeted studying during that hour. As he builds up strength in particular section areas, he'll also likely increase his studying stamina.
For more great tips on how to get the most out of studying for the SAT, I recommend reading this article.