Dont stress and continue doing what you are doing. I would avoid flash cards and worksheets if he finds them boring.
Also, read to him everyday. As you read to him, point out letters here and there and words (dont make the whole book about it). Run your finger along the text as you read. If a word is repeated over and over, or in big special print, tell him what the word is and have him read it on the following pages.
Work on letter sounds in casual conversations. If you go for a walk and see a bird, say, "Why, thats a bird! Buh Buh Buh bird! Bird starts with a B. buh buh Bird! What else could we see or hear that starts with a B sound?" As you see other B words, or encounter other B words in your converstation, note them and compare them to Bird.
Also rhymes - There is a dog. Og. That sounds like frog. Dog Frog Dog Frog. Can you think of anything else that sounds like that?
Another idea - get foam paint brushes and a bucket of water. Go outside and write letters with water - at least get him so he can do the letters in his name.
Here is another good idea - I love the ideas in this blog: teachmama.com
Play with letters and sounds and have fun with it. If he likes cars, drive a car through his sand box to make a letter.
Finally - he may know/have obsorbed more than you think. He may actually know many of his letters but just be too stubborn or disinterrested to let you know.