Forcing young children into academics almost never works out. Good preschools and most kindergartens have play-based learning because that's how children learn the best. I don't know what your rush is, but please re-think this - you are setting up a battleground and turning learning into a chore he resents.
Go to your free public library at least once a week. If they have a program there (puppet show, game day), great - but even if they do not, do 2 things:
1) have him browse the books on display by the librarian, and choose one.
2) choose a topic he likes (dinosaurs, insects, planets, farming, cooking....) and show him how to find that section of the library. Start with the computer, and go to the "search" function, and choose "children's library." Then have him type in the topic with your help. Make it your job with his help - not his job to do. So, if he likes "farming," then say, "Hmmm...farming starts with F" and then make the sound, then make a game of finding the F on the keyboard. Don't quiz him or drill him - just make it a treasure hunt since the keyboard is not in alphabetical order. After the F, find the A, then the R, then the M and so on. Don't get frustrated, just have fun - if it's YOUR fun and he's joining you, that's better than yelling at him that he doesn't know what an F is and he's taking too long. Then, when the farming books come up, write down the Dewey Decimal numbers on a piece of paper. Then go through the treasure hunt of finding the numbers in the stacks. Just have him find the first number. So if agriculture books are in the 630s, have him find numbers on the signs that start with 6. Stop there. After that, YOU find the 630s and then have him pull out a few books, while you read the titles for him. He can sit on the floor and open a few up, choosing what he wants. Don't influence him or push him - make this entirely his choice. Read to him, and when you get home, read to him some more, every night before bed. When you come to a letter that is the first in his name, point it out. That's all. Focus on the joy of reading!
Now, take him to children's museums or zoos and read the signs/descriptions together. Whatever he wants to find out more about, make that a game. "Okay, you want to learn about the lion? Lion starts with L, so we have to find a sign that has a big L on it. If he finds the Lemur instead, you can comment, "Good job finding the L! But this animal is the lemur." (Point to the letters.) "Lemur starts with L-E. Lion starts with L-I. So we need to find the I right next to the L. I'll look." (And then hopefully he will help you. If he doesn't, just go learn about lions.) But don't spend the entire day searching for letters - it's really about seeing the exhibits and animals, and making learning and exploration FUN! You have to be excited about it, not using it as a lecture opportunity.
Get some simple board games for his age, and make time every other day to play together. He'll learn to read little by little just to advance his play.
Beyond that, leave the teaching to the teachers. I promise you that he will know all his letters by the end of kindergarten. Don't feel you need to teach him everything so the teachers don't have to - especially when you find teaching so frustrating.