I echo those who posted that I wouldn't worry too much about your little guy not knowing everything before he gets there. Our difficult child could read half-way through kindergarten, but our easy child showed little or no interest in learning the alphabet, numbers or shapes FOREVER!!! easy child just finished 2nd grade and now is as good a reader as difficult child was at her age - they all get there in their own time.
I taught both kids the alphabet by using the book "Dr. Seuss' ABCs". Big A, little a, AAA, Aunt Annie's Alligator, AAA. Very, very repetitive, but fun and quirky at the same time. We read it every night - along with another, different book so they didn't get TOO bored, until they could at least recognize the letters. Flashcards are a good idea, too.
Again, I wouldn't stress too much (says the woman who over-thinks everything) although I know that's easier said than done, especially with our difficult children and the school districts.
Good luck.