Be sure that your making the learning fun, not pushing. Sometimes things get pushed so much kids shut down and just don't know from either to much repeatition, or because they think they are letting their parents down.
Associate the letters with words things he likes, I know there are many flash cards out there, but think about going around and getting pictures from books, magazines, or printing from online, of his favorite things.
If he likes Thomas the Tank Engine, great way to remember T, if he likes Elmo great way to show him E.
If he likes Cookies, or Hot dogs you have H and C.
There are many books out there with letters. Each page having pictures and words for specific letters. You can get books that have animals, or ones that have a variety of images. Reading different books like that with lots of color can make it more fun.
First go over the pictures, getting him to recognize the pictures, and or colors, eventually he will learn the letters that go with them as well.
When it is a fun game, and you are over the top happy whenever he is right, and never sad but cheerfully telling him the correct answer when he is wrong he may begin learning easier.
or you can try to sit with him at the computer at sites like this, and play the game of animals that start with letter.... it might help if he likes animals.