You are rightfully trying to leave a highly controlling abusive relationship.
Talk to family and get a plan in place.
Can you pack and leave without telling him?
This is not a good man.
You owe him NOTHING.
Abuse changes everything.
Get paperwork, finances, pay slips, passports, pensions etc., together and give them to trusted family.
Pack up your valuable sentimental things, childs favourite toys.
Give them to family.
A letter is fine.
Be prepared to involve the police.
He has deliberately controlled you for years, so you owe him nothing.
Get out as safely as you can.
Speak to Women's aid for advice and support.