The most important skills that a preschooler needs to have are not academic, they are social and self-care skills. Can you leave your son with a babysitter or playgroup without having him cry excessivly? Can he put his own shoes and coat on? Can he express his needs to adults in a manner that they can understand (not just you and your husband, other adults too). Does he have problems with biting, hitting, kicking, or aggression? I have known one child who was kicked out of preschool for repeatedly biting the other children, it did not matter that he knew his alphabet and colors, he was making school an unsafe environment so he had to go. Learning to share, play nicely with others, communicate with teachers and peers, listen attentivly, and take care of some of his needs independently are some of the most valuable lessons that your son will learn in preschool, and these are skills that he will use his whole life long. You can start teaching him these lessons at home, but it is also good if you can get him involved in a playgroup, library story hour, church group, or gym class where he can interact in a group of children.
As far as academic skills, don't push him too hard, you want to encourage a love of learning. You should not ever force him to learn colors or numbers or letters if he is not in the mood for it, just keep his lessons informal and fun, he should not even realize that he is learning, he should jsut think that you are playing together! Like when you play with blocks you can ask him to hand you the blue square or the yellow triangle, or the block wiht a B on it if you have the alphabet blocks. He will be most interested in learning the letters of his own name because they will have the most meaning to him, so you can point out on a sign or a book, "look, that word has an M in it, just like your name, Mikey." When you are driving you can comment on signs that you see "There's a stop sign, s-t-o-p, that means I have to stop the car, isn't that stop sign a cool shape? It's called an octagon." When you are baking have you son help put in the ingredients "Now we need to add 3 cups of flour, one...two...three, OK, now 2 teaspoons of vanilla" That kind of everyday learning is going to teach your son more effectivly than any worksheet ever can. Good luck and have fun teaching your children!