The key to your problem is the sentence that said, "He is very bright and has no problem asking tons of questions about what interests him."
Letters and numbers, shapes and colors don't interest him right now, and the more you push it upon him, the more resistant he is going to be.
Instead, read to him about what he's interested in. Is he interested in trucks or bugs or whatever, get books out of the library that talk about those things. Point to the pictures and ask him what it is. Say, "Show me the blue truck" or "How many bugs are in the picture?"
And help him explore anything that interests him to the extent of his attention span and interest. If he likes bugs, collect them and learn all about them. My son had a little plastic bug cage he could carry and he would catch a bug. Then he would bring it in to show me, and then would get in our bug books and look up what it ate, how to tell whether it was a boy or a girl (applies to crickets, grasshoppers, etc), where it lived (grass, bushes, dirt, etc) so we could make a nice home for the bug in the little cage. Sometimes we would draw a picture of the bug for grandma and he would need to label it, etc. He was allowed to name the bug and then keep it for one week, and then he had to set it free, and then he could catch another. We learned about worms, ants, crickets, spiders of various varieties, a praying mantis, etc.
My son also liked trucks alot, and we visited the fire station and a rental place that had the backhoes and stuff. We talked about their colors and I took a picture of him sitting in the wheel of one of the huge dump trucks. I helped him to count the dump trucks, the backhoes, and all the trucks, etc.
We also collected rocks for a while. We went to a rock quarry and learned about minerals. We made a seashell collection and a leaf collection. We learned the names of all of the plants in our yard and planted some seeds. All of these things we had to read about and counted, etc.
We also cooked alot. In cooking we measured and counted, and I would read the recipe out loud so they could internalize that reading was helping me understand what to do. Eventually, I would ask them to help me sound out a word on the recipe card, such as "cup" or ask them to tell me, "How many ... do I need, again? What does it say right here?"
Learning isn't limited to learning the alphabet and numbers, its about the world around us and how it works. In the course of learning about the world around us, we can read and write about what we see, learn the history and science behind what we are discovering, and learn how to count or compute what when we need to.
Put your creative energy into exploring his interests and helping him learn all he can about it, and move on to the next thing when he's ready to move on. You won't believe how much he is learning, and even more important, the memories of time spent together. My son is 21 now, and I can still remember so many happy moments spent with him and my girls when they were all little that I am so grateful I took the time and effort to do. It doesn't come back again. Your time is now, enjoy it.