A I'm sure that you're relieved that your son's reading has improved - I think you should also discuss his spelling progress with his teacher. As a parent, it is easy to look at a piece of your child's writing and only notice the errors. There are ways of helping, but it is sometimes difficult if children feel pressured. You should comment positively on his writing as much as possible, responding as a reader, rather than a corrector.
As a teacher, I found it reassuring to count the number of words in a piece of writing, then the number of errors. I often discovered that the percentage of errors was lower than my initial impression. I would also look at the kinds of errors a child was making. Were their spellings mainly phonetic - such as writing "biyootiful" for beautiful or "caruts" for carrots? If this was the case, and children could make fairly plausible phonetic attempts, then I would encourage them to start looking more closely at words, because spelling in English is about visual patterns, as well as sounds. I encouraged children to practise their spellings with a spelling log using a "Looksaycoverwritecheck" approach.
In my research into spelling, one of the main problems for children who had difficulties is that they tended to see words in isolation. Looking at words with similar patterns, for example, right, sight, might, and so on, or words ending in "ion", as in station, nation, or words with "pp" in the middle: hopping, shopping or words with common roots like medicine and medical is most helpful for these children. These words can be collected in spelling logs. As soon as possible, help your child to use a spellchecker on the computer.
You can also purchase CDs such as Starspell (for younger children 6-11) or Word Shark (for children aged 9-14) which provide systematic but relatively enjoyable instruction in a games context.
Use a Look Say Cover Write Check game, available on Ambleside school website: www.amblesideprimary.comambleweblookcoverlookcover.html, or visit the BBC Spellits zone for a variety of games: www.bbc.co.ukschoolsspellits. You will need to download Flash4 (free) on to your computer to use these interactive games.