There are also sets of blocks with numbers or letters on them that could be fun, and of course matching the colors or shapes of the blocks
What about getting him into board games like candy land and chutes and ladders? Coloring? Sensory seeking things like spinning chairs or rocking boards?
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My son loves the alphabet; he has alphabet books, flash cards, and puzzles. He also likes associating things with their letter, like holding up the letter B along with a ball, or a C along with a cat plushie. Recently he started enjoying drawing more if we would help him draw a letter along with a picture of something with that letter. And he loves this alphabet board: https://a.co/d/d9XBlc7