My SIL was in a very similar situation with her ex and my niece.
The police let them know when he was being released and what his release conditions were.
We are in Scotland but expect it will be similar, If they haven’t already, they will want a formal statement from both your son and you. When you have a bit of time, it’s worth jotting down some of the details of what your son told you, and what you said or did when you spoke to the police, while it’s still fresh in your mind. It could be months, or even longer, before it gets to court and, if he pleads not guilty, having those notes may help you remember the details accurately.
You may also need an appropriate adult for your son. As you are a witness, you may not be able to fulfil that role yourself.
I sat with my niece as her appropriate adult while she gave her statement, and heard a lot of what had happened for the first time. It was honestly one of the most difficult things I’ve ever done, try to get someone he can trust. Surprisingly my family were angry at me for respecting my nieces wishes and not sharing the details of that statement until she was ready.
And don’t underestimate what abusers can do to try to manipulate you once they’ve been exposed. Equally, don’t underestimate how adamantly some of his family and friends may disbelieve your son. They may be in complete denial, but it’s something to be prepared for.
Not sure how to finish this post other than to say I am so sorry this has happened to both of you.