Invite the child to a lesson at a rug with the Montessori sandpaper letters. Ascertain mastery of the phonetic sounds by showing the child one sandpaper letter at a time and asking him what the sound is. When the child shows that he knows all of the sounds, he is ready to put the sounds together to make words.
Put one consonant, such as /m/, and a vowel, such as /a/, on opposite sides of the rug. Point to each one as you say its sound. Start pushing them closer, one at a time, saying each sound as you touch it. As the sandpaper letters move closer, say the sounds faster. By the time the two letters have met in the middle of the rug, you should have the two sounds blended into one sound, /ma/. Keep practicing with different consonants and the same vowel.
Add a third letter as an ending sound, when the child feels comfortable blending two sounds. Change one consonant at a time, to allow the child to keep practicing. Use real words.
Do dictation with the child. Say a word and have the child figure out which letters to put together. Use objects or pictures of words that follow the consonant-vowel-consonant pattern, such as "cat," "hid," "cut," "dog," "bed," as a prompt. When the child demonstrates he can spell some words, he can then move to using the Montessori movable alphabet, which is less bulky.