Strategy:
Behave perfectly, much as you may not want to, and be able to prove you are.
Beware - he has already started to try shifting blame to you with his 'are you prepared to fight for your family' message.
He'll be spreading lies about you to family and friends, suggesting it's because of you, that you're trying to keep the children from their daddy.
Let family and friends know what's happened. Get in first, own the narrative he will be trying to spin. 'I never thought this would happen to our family, but sadly DH has decided to leave me and our children for a teenage girl, and has gone back to live with his parents.'
And IF you feel confident of support there, tell your line manager.
Facilitate him seeing the children.
Do not vent your feelings to them.
Keep records of everything.
Get a dedicated email address and engage with him only through that, and solicitor letters.
Texts, phone calls, messenger DMS etc are too ephemeral and informal. If he attempts contact there, reply via email, and be vigilant about any attempt to contact the children behind your back.
Right now you want this all to go away, for it not to have happened, for everything back to normal. It would be the easiest thing to forgive, have him back, and you're probably wavering - and he KNOWs this.
I wonder if your success has put his nose out of joint and he wants an adoring girlie on NMW to make him feel superior?