Find an alphabet memory match game for him to play. He can also match magnetic letters or numbers to their mates. Some stores carry alphabet macaroni and cereal, too. Alphabet and number puzzles are easy to find at toy stores and educational supply outlets, and may help reinforce the concepts. In addition to these, you may want to invest in dominos or other number matching games.
Books are another great way to introduce concepts.
Read and share alphabet and number books with your son on a regular basis. Your local library may have a selection of alphabet and number books, too. For more ideas, talk to your son's former and current teachers. They may have some ideas for you that will help him make great strides. Although it may take time, he will eventually learn the numbers and letters.