Also, in Scotland... Not England.. 16 is a legal adult. So by God he should know how to make a meal for himself now and then.!
He may legally have reached the age of capacity but he’s developmentally not an adult. If he went to live with his dad you (and his dad) still have a legal duty to support him financially while he is in education or training, I’d also argue there’s a moral duty to support him into adulthood whether that’s with him living with you or not.
This situation didn’t occur overnight, he didn’t wake up one morning and decide to be difficult, how would you normally set boundaries and discipline your son? How much of the abuse did he see in your relationship? You both had significant trauma and need time, help and support to recover, and he may need to live somewhere else for him to be able to heal. Not as a punishment for his behaviour but because you both can’t heal in the same house.
You sound incredibly angry - how much is anger at your son and how much is anger at the abusive ex, or how much is determination not to let it happen again is anyone’s guess, but it would be better for your son not to be living in such an angry environment. You keep saying he’s an adult - treat him like one then, talk to him about your expectations of him in terms of keeping the house tidy, or making meals etc and don’t keep picking up the slack - if you want him to behave like an adult then treat him like one and run the house as a team, stop paying for expensive holidays etc and let him earn his way (or not). Set clear boundaries and negotiate with him as you would any other adult.
Or you could remember he’s still growing and has had a traumatic few years, he’s watched you being abused and has lived with you being prepared to accept that and live with it for a period of time. His recent male role model has been abusive. You don’t stop needing your parents because you turn 16, you don’t stop needing to be parented at 16 but the form that takes changes.
You went from being you two together to introducing someone else into the home, which is tricky enough for teenagers, and that person then abused his mum, and he had to live in a home where his mum was abused and was powerless to stop it happening. It’s reasonable to think he might be angry and frustrated and may have lost respect for you - you can work with him to earn that respect back, to help him with his anger. Do you know how he feels about your abusive relationship? How he makes sense of that and how it came about? Have you talked to him about what it was like for you and heard what it was like for him? I wonder how much he might blame himself for not being able to protect you, for example.
His behaviour isn’t ok, but neither is yours and the only person you can control is you.
Counselling can help but what he really needs is safety and stability in his life. If you can’t give him that, let him stay with someone who can. If you know his dad can’t or won’t, and you can’t or won’t then who knows what’ll happen to him, legally an adult or not.