I would assume this evaluation is pursuant to the local school's obligation under Child Find. An evaluation has to come before a determination of eligibility for special education, then after a determination of eligibility (assuming eligibility is found) a meeting to draft the IEP is held. So no, what is scheduled is NOT an IEP meeting and only an evaluation.
You do not have to consent to having the child evaluated. Did you sign anything? It would have been a consent for evaluation form. If you did, you can reach out to the school and revoke your consent. In extreme cases, the school could take you to court (due process) to order the evaluation, but I don't think they would do that if you explained to them you are in the process of getting him into a private school.
I don't know what ESA is and so can't speak to how funding would be affected. I would suggest reaching out to that process and ask that question directly to them.
This level of disability is not my expertise, so hoping others will chime in to correct or add to my answer.