The best way is just to reniforce through everyday activites. Hands on is the best way to learn.At the grocery store, identify colors of food, counting produce, and letters that items start with. Driving down the road, at home, anywhere really. Make homemade flash cards: write number 1, add one sticker, #2, two stickers. Make alphabet flashcards using photos of family, houshold items or pics from magazines. If you think he would like it, you can buy preschool age workbooks at Target, etc., but things he's involved in will help him learn better.
***also- Do things to increase his small motor skills to help with cutting and penicl holding. Get some kid safety scissors and let him cut up magazines. Playdough, puzzles, legos, lacing cardsand stringing beads are good for that.