When the child is learning to say each vowel sound, have the child first practice the sound in isolation (by itself without any other sounds) and then in nonsense syllables (combine the sound with different consonants like “boh, yo, low, go”).
Once the child has mastered that, he can start practicing the sound in single words. This worksheet pack includes one worksheet for each vowel sound and each worksheet contains 20 words that use that vowel.
Start by having the child say the words on the worksheet after you. Once he is able to do that consistently, point to each picture and ask “what’s this?” an have him say the words on his own. Once the child can say all of the words on the worksheet, have the child say the words in phrases (like “my ___” or “I see ___”.
Then, once he’s mastered that, he can move onto saying the words in full sentences that he makes up. Just ask him to use each word in a sentence. You can also just move on to a different vowel once he gets to this level (if he’s not able to make up sentences on his own).
It doesn’t matter what order the child learns the vowels in so I recommend starting with whatever vowel he finds easiest. This will keep him from getting frustrated and will help you make quick progress right from the beginning.