What worked for us in a very similar situation was to contact the principal of the school and say, basically, what you said in your post. That is, this is where are DS is and we know that the teacher will have a broad range of abilities that he/she will need to meet, and these are the solutions we have thought about; we'd really like your advice about what you think will work best for DS next year. Just lay it all out on the table and ask politely for their help and expertise.
First, this allows you to start your new relationship with them (since you already have a relationship with the regarding your DD) with a compliment and and an acknowledgement that they have years of experience with education in general. It gives you an opportunity to introduce them to your DS. And it gives them the opportunity to help you, and helping is something everyone intrinsically likes to do.
And then sit back and see what they have to say. In our case the school tested DS and observed DS and realized that differentiation was going to be a very involved thing and it would be unreasonable to expect a K teacher to make all the accomodations necessary to meet our DS's needs, so they suggested he skip K.
If nothing else, though, going to the school and asking for their help with get a dialogue started about your DS in a relatively non-threatening manner. In any case it will be a longer process than just one meeting and will likely continue over the years (I k now you know this). At least you'll get the process started and see if you need to start thinking of other options.
