I would agree that Wilson is excellent, and my top cost-no-object recommendation, but you definitely want a certified tutor. Barton is also good, but expensive for the home program. AAS is essentially equivalent to Barton as a home program, and the same idea as Wilson, but scripted (so you don't need the specialized training), and cheaper than Barton.
If you think a summer of intensive tutoring will do it for him, it might be worthwhile to spring for the Wilson tutor, as a trained one-on-one should be able to make the accelerated personalization adjustments easily enough to provide quick but comprehensive coverage of most spelling rules, zooming in to spend more time on trouble spots.
If he has more pervasive spelling deficits, or if cost is a factor, I would go back to AAS, and deal with the customization yourself. One way or the other, the best outcomes will be with someone working with him one-to-one.
