I don't think the advice to message him is very good advice tbh.
What he did to you was about power, and you sending an angry message to him now will validate that for him, knowing that it is still effecting you won't make him feel bad but any, or no, response will likely make you feel worse.
Whether you report it or not is a different matter as well, and something I might be wary of.
In the most gentle way possible, its pretty unlikely that he will get charged, let alone convicted, and you need to think about how that would make you feel.
It may be that it will feel good to finally report it and get your story written down officially, or it may have a detrimental effect on your life if it doesn't go further forward.
I guess what I am trying to say if that nothing he will say at this point is going to make you feel any better, so you need to think only about you, what you need, and how to get yourself through the shock of seeing him again.
Have you had any rl support for what he did to you? Would you consider a support group or counselling?
Fwiw I believe you, and you have absolutely no blame in this whatsoever 