How do you get your 4 yr old to remember what letter of the alphabet is what?

I have a 4 yr old son and he can't seem to remember what letter is what. Even though I go over it with him 1 min later he can't remember or he say's he does not know and starts to look sad. I don't know if he is just not listening, being lazy, ETC...( I relly want him to learn and excell in school.)
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