Download worksheets from online reading resources. There are several sites such as Education.com, Have Fun Teaching, ABCYa and TLS Books, where you can obtain materials for free. Look specifically for "vowel sound" worksheets. Focus on worksheets that show an image the child can sound out. For instance, on Education.com, there is a worksheet for "Long Vowel Sounds" which instructs the child to circle the images that contain those sounds.
Encourage the child to tell you what pictures on the worksheets are allowed. Or, if they have words, have the child read the word aloud. Using this worksheet example, instruct the child to pronounce the first two images which are "rake" or "alien" aloud as they circle them, since they both have long "A" sounds.
Notice where he correctly and incorrectly pronounces the word. For instance, using the worksheet above, listen and see if the child performs the third and fourth words on the sheet "snail" and "grape" correctly. The child might mispronounce grape for "group," "gripe," or "greep" for instance.
Instruct him to repeat the word by sounding out each syllable. If he is unsure what a syllable is, have him tap his hand on the table to get the gist of each beat. For instance, on the worksheet, the 2nd grader may be pronouncing the sixth word "doughnut" as "donit," "donat" "danut."
Correct the child on the syllables he is pronouncing incorrectly. If the child pronounces "danut" for instance, instruct him to form a circle with his mouth and perform the "o" sound several times until the "do" portion of "doughnut" is correctly pronounced.
Continue to work on the vowels. Download other worksheets from the aforementioned sites. Many of them work on grouping like vowel sounds together. For instance, worksheets might focus in the long "a," "e," or "o" just to name a few. Repeat the steps above with each worksheet.