Collect up all official documents that are yours and DC especially birth and marriage Certs and passports
Open a bank account COMPLETELY separate from him (inc different banking group) as otherwise you may be subject to it being frozen.
Get child benefit, tax credits, utility bills transferred to your name if not already the case. With benefits make sure they're paid into the new account.
Withdraw at least 50% of money from any joint accounts.
Gather info on his income inc pension/shares.
Pp are right DO NOT leave home.
Check what benefits you can get when he leaves via eg entitledto site. Amounts not always accurate but which benefits is. This then let's you know which agencies to contact to get application forms for these. While you cannot claim until you officially split you can get the forms and start filling in and collecting evidence required which can take time.
When he leaves inform school so they can support DC and you. Also so you can be clear who is meant to be collecting them which days, update emergency contact info, if you choose it may be easier for them to send him letters re parents eve etc
Controversial but I'd also advise he hand over keys to house when he moves out. I've known of several situations where the ex has entered the former marital home and taken furniture, valuable items even money, also planted spying equipment.
Be careful what you say and how you say it, put nothing in writing (inc texts) that can backfire. Seems easy now not so easy once emotions are running high.
Look after yourself re eating, sleeping, stress and don't hesitate to go to dr if you start to feel you're struggling.
Sorry you're going through this 