DS1's teacher in yr1 taught them 'magical spelling', which has been fabulous for him. First they look at a word, and think about how it's spelled. Then they close their eyes and imagine the word in their head. They imagine it in a certain colour - maybe large purple bubble letters with yellow spots on - the detail is all part of the system.
Then you get them to look carefully at the shape of the word: you can actually draw the shape of the word. (can is just a rectangle, cat has an up bit at the end, rag has a down bit at the end - does this make sense?)
Then you look at anything particularly unusual about the word - i.e. silent 'k' in knife, etc. If a word is not phonetic, get them to pronounce all the letters: WED-NEZ-DAY, etc.
Finally get them to 'take a mental picture' of this image in their brains to remember.
Then practice, practice, practice.
The other thing that helps of course is loads and loads of reading, so the spellings are more familiar anyway.
Also if you have a tough batch try putting them up next to his bed, or on the wall next to the dining room table.
Hope something in there is helpful! How old is he btw?