This is a tricky question, since your DS will be going to a GT charter, where presumably things will be advanced. When my DS was going to a regular local school, I went well in advance and told them about my son. But when I switched him to a GT school, where they were used to kids like mine, I didn't feel the need to do a whole lot ahead of time.
I guess if I were you, I would want to know about how the school differentiates. Since it's a GT charter, I would be hopeful that they would already be doing typical 1st grade reading, and there are probably a lot of kids that will be in the same boat. But the only way to find out is to ask. (E.g., my son was tested at this level. Will there be groups of other kids learning at this level as well?) Do you happen to know if the psych meant that your DS should do typical 1st grade reading or 1st grade reading at the GT school, which is likely to be more like 2nd grade reading?
Also, FWIW, my experience of advocating ahead of time before K didn't do a whole lot of good, since the teacher wanted to see for herself what DS was like. This delayed differentiation for about 6 months....
