Inappropriate teacher?

Hi IM’s, I need some advice e about a very delicate situation… My son is 9yo and on the autism spectrum, meaning that he’s quite socially and emotionally inappropriate, something that we’re working on with him every single day and have spent years and 10’s of 1,000’s of dollars trying to teach him through therapy.
Two years ago, his one and only close friend went overseas for 6 months, so we let him have his own email address so that they could keep in contact, with the strict rule that he wasn’t to give the address out to anyone. My husband and I intermittently monitor the email to make sure there’s no dodgy emails and our son knows this too.
About a month ago, he gave his teacher his email address – his female teacher, a mother of 2 young kids of her own and I’m guessing in her early 30’s. I have no idea the conversation that led up to him doing this, I don’t know is she asked for it or if he offered it, but the first contact made was from the teacher to my son. His communication is a little impaired, so I never get relevant information from him and the more I question him, the more difficult it is to understand what happened.
Fast forward a month, and they’ve been having quite a conversation over email, without my husband and I being made aware of it. The teacher isn’t being inappropriate in the content of her emails, mostly sending our son jokes and links to funny, kid friendly blog posts, but my son is being inappropriate with the teacher. He’s telling her that he loves her, and what a lovely person she is, and a great teacher and he’s the one asking her for contact.
Regardless of how this came about, it has to stop, but what do I do?? I have to believe that the contact began innocently enough, I say “have to” to give the teacher the benefit of the doubt that her intentions are all above board, but I feel that the contact itself is inappropriate. We absolutely need to have a conversation with our son and add more strict parental controls to his email address, but should I approach the teacher, or the principle? If I approach the principle, then this will become an official complaint of inappropriate interaction with a student, and that’s a serious and awful thing to accuse a teacher of if the intention was innocent. On the other hand, she’s the adult in this situation and in a job where her behaviour is constantly under scrutiny and is governed by a strict code of conduct that she must adhere to, and this interaction is definitely a breach of that code.
I’m so freaked out and confused and really don’t want to get a teacher in trouble if it isn’t warranted, but at the same time, it must stop and she must be made aware of how inappropriate this behaviour is with a young and vulnerable child.
So what should I do?