How much does your dc read? Kids who read a lot can generally spell very well. I think it's something to do with them getting used to the sight of the words through reading. Apart from that, there are loads of fun workbooks available - I've found these a lot more useful than websites/CD Roms as they involve writing the words out which also helps with spelling.
It depends on what kind of things your child likes. A lot of the workbooks are very bright and full of weird characters - this works for some kids but mine have found them very distracting (the pages are just too busy). I've resorted to the Carol Vorderman books which are simple, direct and easy to follow. There are no gimmicks, just straight-forward exercises to do.
I think reading is the key though. One of mine has just really got into reading (I found a series of books she likes) and her spelling has improved 100% in just a few weeks. Can you find any books he likes? Some kids prefer non-fiction.