Well done, you sound like you know what you're about to be dealing with, and preparing like this will make you stronger. Be ready for abuse and manipulation tactics to go off the chart. He may cry, beg, shower you with gifts, act like the worlds most perfect husband - all to buy himself time to think of ways to fuck you over.
get keys copied for anything and everything you own - garages, sheds, home doors, etc. so you can't be locked out of your own place make sure you have already removed anything precious you think he might try to 'lose' to mess with you. Stash stuff with friends, just get it out of the house. Get marriage certificate, passports. Copies of all bills, mortgage statements, notes of bank account numbers or credit cards and pension info if you can. Register your 'home rights' with the land registry (I think it's them, can't remember). This means he won't be able to sell the house from under you.
if you're going to be leaving the home, think about how to port things like shared digital services - Netflix/amazon passwords which WILL be changed so you can't use them. Anything in his name you both currently use, assume you'll no longer have access to and figure out what you want/need. You may need to set up new accounts in your sole name.
if you have utility bills in your joint names, close these accounts. You can either reset them up in your own name
if you want to continue paying, or let him. Do not let yourself get into the hell I am where we're both in the bills but he won't pay his share and can run up arrears that I'm still legally liable for. Take half of anything in joint bank accounts or savings out into an account he can't acces. Or, if you feel ballsy, take out all of it, on the understanding that half will still belong to him. It may be the leverage you need when he keeps/sells something else you jointly own of value.
google grey rock method and put it into practice. Do not be drawn into pointless circular arguments. You do not have to Justify, Argue, Defend or Explain yourself- google the JADE method. Follow Your Dicorce Coach on insta who has some great advice and techniques to stay sane, also Lisa Sonni on leaving a narcissistic abuser.
act quickly. Time and surprise will be on your side. If they are narcissistic, they will attempt to thwart you whatever you do, and regardless of what would actually be best for your child, so don't give him a chance to hide assets, drag it out and make life hell.
if you have to interact in person, keep your phone recording in your pocket. Keep a record of everything by emailing yourself at a new account. Assume every password you have is already compromised and change them all.
he will do everything to stop you walking away. Just keep your eyes on the horizon and keep stepping away. It will feel impossible at times but keep plodding, recognising the difference between what in your control (your boundaries and enforcing them) and what isn't (all the shit he will do to trample all over them). Try to avoid conversations in person as he'll just use this as an excuse to start and argument by doing DARVO, do it in writing so you have evidence
I am rooting for you, you can do this. Everything feels impossible until it's done.