I think you should be impressed! It sounds like he's a smart guy, and you have worked very hard with him. He sounds like he enjoys it, which makes it even better! Keep up the good work, and he will continue to excel. Now is the time that you can work with him on colors, numbers, and shapes as well. We made a game out of counting. My daughter loved the windshield wiper fluid to squirt and would yell, "Car raining!" when we did it. So, we would count to 5 for it to happen. Pretty soon, she started randomly counting to 5 in the car, so she could see the car rain.
I used those picture flash cards (mine were Baby Einstein ones) with her as well, because she really liked the pictures and it was fun. There is nothing wrong with that!
Have you seen that "Your Baby Can Read" program? I want it for my next! It's only an hour per day that you work with them, and if they aren't enjoying it, you can put it up and wait a few weeks, then try again. My daughter would have LOVED it. Some kids really do love flash cards and work books. Mine was one of them, much to my surprise. She is a highly gifted 7 year old now (by highly gifted, I don't mean that she is a super cool kid. She has an abnormally high IQ - and is still a super cool kid).
Most likely, he will get to the point where he will ask you a ton of questions.... do whatever you can to answer all of them. Whatever you do, don't let anybody squash that curiosity. It becomes tiring asking so many questions, but when they really want to know things, it's important to do whatever you can to give them the knowledge that they want. I only say this because many MANY parents advised me to limit the amount of questions I would answer. It's very likely that you will hear the same thing, also having a child who is hungry for knowledge.
Good luck, and keep it up! It sounds like you are doing awesome with him!