You didn't say how old he is?
He might need help with phonemic awareness. Or he might just be young still and needs more time. The method you use can also affect results.
My sister is a dyslexia specialist and uses a program called LIPS to help kids who have problems pronouncing the correct sounds of letters. She helps the children learn how to form their lips, teeth and tounge for the appropriate sound.
My youngest had a lot of difficulty saying sounds. (I think her 1/2 inch overbite didn't help). Eventually we prevaled. My sister could have helped but lived a half hour away in Fremont. So we worked on it at home using SWR cards. She is 7 and does fine now. We homeschool, and use a program called Spell to Write and Read ,(SWR),by Wanda Sanseri.I compared it to her reading program and it uses many similar techniques and reading rules. It has flashcards. We hold up the card and say only the phonogram sound, (not the name of the letter.) Also there is no picture on the cards. Both of these differences help with lessening confusion. Only a white card with a black letter typeface on it. For example, I hold up a letter A, and I say "a - A - ah " (the short sound, then the long sound, and finally the ah sound). I say them in exact same order everytime. They are said from most common sound to least common sound. After she learns the sounds of the letters on the cards, we then learn the name of the letter as she learns to form the letter on paper , and subsequently spell it in words and write the spelling words in a journal. From all of this, she learns to read! It's an amazing program, and no struggling or tears like the other reading program I used with my firstborn. However, you will need to tutor yourself by reading the coresponding manual, or go to training classes to learn how to use it. If you know someone who homeschools and uses it they can be a Godsend, because I think just using the cards alone, without all the other stuff, if used correctly, can help kids get past many reading hurdles.
Whatever you decide to use, be patient and know that he will get it eventually. If he is 7, 8, or 9, in the public school system and still struggling with correct letter sounds, try a dyslexia or other special ed tutor who uses the lips program (barton program)or other equivalent program.
If he is just young and not remembering, some kids do need more time than others. My youngest, (with the overbite) took twice as long as my other two to memorize letter sounds. It was like pounding it into a wall! It seemed like she had to be at a brain maturity level before it could sink in. Once her brain was "ready", then she indeed learned it. Sometimes vitamins can help with memory. Thiamine , glutamine , lemon balm, Cod liver oil, are all documented to help memory.