That's the first assessment they've done for your son? Wow! That seems kind of slow, but maybe typical for kindergarten? DS has done at least 3 thus far in grade 1 .

Maybe ask your teacher if you could send some books from home? We do that often.

My DS also loves to devour books "below level" for him too. It's like brain candy. Like Geronimo Stilton or Goosebumps.