Could you ask the preschool teachers who have expressed these concerns to contact the K teacher rather than you? It might hold more weight coming from another teacher, even if it isn't a certified teacher.
I agree that this would really be the way to go, and sooner (now) rather than later, since any adjustments a school would need to make might take longer to line up than you would expect. Schools will definitely give more weight to the opinions of trained, experienced educators than to a parent who they do not know. That being said, after the preschool teachers contact the new school, I would contact the principal, say that you're concerned about the fit between your son and the classroom because of what you've been seeing, and see if they will do ability and achievement testing over the summer (probably now or in August) so that you all can get a better sense of what you're dealing with and whether any accommodations will need to be made, and if so, what might be likely to be helpful. It never hurts to play the "I'm a bit confused, and I don't want to come off as saying that my child is the most brilliant child ever, but could you please help me figure out how this is going to work?" card. It might sound jaded, but positioning the encounter as if you are allowing them to help you can often work better than going in with a list of why things won't work (not that you would do that).
Is yor DS old for grade or young for grade? Do his friends tend to be older or the same age he is? If he gets antsy in school when he is not being challenged do you see his behavior improve in situations where he is being challenged? All of these are things that the school might want to take into consideration.
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