How does your son learn? If he's an audial learner, you might go the "schoolhouse rock" route and have songs that explain how to spell different things. If he's a visual learner, start writing out the words in different colors (the letter combo you're working on the same color in each word, with the rest of the word in black or blue), so he can see the rule in action. If he's a kinesthetic learner, get some index cards and make up the words using the letter combinations you're working on, letting him make the word (kind of like a puzzle). Also for kinesthetics, you can write the alphabet in sidewalk chalk (outside or in a basement?) and have him jump to the next letter.
IHTH!