Invite him to copy the letter shapes in plates of flour, sugar, salt, or whipped cream. He may even want to copy the shapes using clay. Matching is also a good activity for a child this age. 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.