Several things jump out at me.
-It looks like you feel that the school/principal/teacher did not give your son a fair trial in K because they are offended at early entry and high test scores. This may be true. That is not a situation any kid should be in, and I would try to change it before trying school again, either by going elsewhere for K next year or by developing a good relationship with the school. Stay open to the idea that your impression may be mistaken, too.
(OTOH, I have zero advocacy experience, so I would be looking for lots of advice on how to do that in the coming months.)
-As others have said, a broader range of experience will help tremendously in the next year. You have great suggestions here. Kids aren't supposed to enter K knowing how to read or even necessarily the alphabet or how to write their names. Don't worry about this if he's getting plenty of quality time with an adult, playtime with other kids, and a chance to learn to follow a group. We really liked our preschool gymnastics class for this; they explicitly taught lining up, following directions, separating from parents, etc., and it only took an hour a week.
-Finally, I'm opposed to the use of flashcard drills in young children because they were used on me. My mom tried out some addition cards with me when I was too young to understand why (probably 5). I couldn't figure out why she was asking me *again* the answer to something I had just answered. I concluded that the point must be to recalculate each time from scratch. This made me slower and the decrease in speed made her worried, so we did more drills. To this day I don't trust my math facts recall and recalculate every time, never passed the stupid timed tests in elementary, and generally doubt myself on things I just remember without corroborating information. It's also the earliest and most vivid memory I have of trying to divine the motivation of the person interacting with me because they had ceased to make any sense.
Whatever the cause, it sounds like that K classroom this year would have been a bad fit. Make sure your son knows you don't blame him, and give him the experiences he will need to grow at whatever rate is right for him. It's hard to go wrong at this age if you follow the child.
(by which I mean his interests, level, and needs - learning he doesn't get his way all the time can be a way of following the child, if it's what he needs to learn next.
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