If the K class is still not fitting where he is academically, maybe he'd be better off in a play based preschool where the structure is more relaxed and he'll be able to do whatever interests him at his own level at school, and you can afterschool him on academics.
I think a four year old reading at a third grade level is going to be happier painting and building than doing letter worksheets and counting to ten. He can write stories and use the blocks to practise division if he likes. And he won't be held to the behavior standards of a five year old K class. I don't know about a young 5's class, but my DD's K class sits quietly all day long.
I would never have described DS as "hyper" prior to being told about this. He will sit very calmly for hours and play alone at home.
Is what he's focusing on at home something he chose himself? That makes a difference.
