How old is he?

I'd disclose your reports. Not as a "he's gifted," but as a "we've had concerns about his development, and here's the information we have." Do it as you request and IEP evaluation for him. You'll want to start that asap as it's a long process.

How old is he? With a half a year of K already, I wonder if going into first might be appropriate. Certainly bring with you a report from the previous school of what he's already mastered.