Each child is different. I have worked with children who didn't want to read until 6 years old but then took off full force reading. Maybe the key is to find out about his day during preschool, what they are introducing? Some preschools are real good at introducing and teaching but not all. Maybe there is some reason why he isn't interested in reading just yet. Starting with flashcards or word walls of pictures and words, then go to picture texted books (these books have pictures in the middle of the sentence) also you need to find out what the child likes best, dinosaurs, animals, cars, etc, and go from there to create or design your curriculum for reading. Even though he goes to preschool you can still teach him the extra's at home without pushing him. I have had children who went to preschool all day and just colored so when the child come home the child didn't even want to see a crayon, the child would throw a crayon, or swipe the paper off of his/her desk, that gave me the hint that there was way to much coloring going on in the child's life.
Maybe if you slowly introduce word cards, flashards, that interest the child with the child's favorite charachter on them, use letter tiles or magnetic alphabet to start spelling the child's name then go from there with small words.
www.starfall.com can be a big help as well. That is what I used with BK LR program, and my 4 year old can read and my 3 year old can read small words like milk, etc. we haven' t gotton to read an entire book yet but we are working on it. Also labeling things in the child's room and all over the house on items will help introduce more words.