I feel physically sick at the thought of everything that will go on whilst my son is not in my care. My husband will allow his parents and extended family to take my son wherever and do whatever and there will be no regard for his safety, and he will learn so much unhealthy, toxic stuff. I can’t understand how on earth it is better for me to divorce my husband than stay for the sake of my son?
He is your son's father - this man is his father. It is too late to change that.
He is not abusive, but he is by no means a perfect father. He is a tosser really.
What is making you sick is an illusion that you can somehow micro-manage the relationship between your son and his father by staying close. You cannot bear that your son and his father will have a relationship outwith your control because you cannot stand your husband and the way he parents. This is a control issue, and it is your issue. The only person it actually benefits is you, even while you imagine it is best for your son.
If you stay together your son and his father will STILL have their own relationship. He will STILL parent him and have an influence because he is your son's FATHER. Your son will wish to LOVE his father whatever you do.
Your son will be 100% of the time in a toxic atmosphere, partly perhaps because of his father, but also because of his mother and father's relationship. 100% non stop, a horrible toxic atmosphere, every day of his childhood. Do not imagine you can hide anything from a child.
You need to let go of trying to control, and accept that if you split, you can at least show your son normal behaviour in your own home. He needs that in order to grow into a responsible, normal adult. He needs somewhere safe, even if it is only for 50% of the time. He needs a stable base. That is the only way he can learn to see his father's behaviour for what it is, if he has something solid to measure it against.
Whatever happens, your son will have to make his own relationship with his father, and you cannot dictate that. It won't be perfect by your standards, but it will be THEIR relationship.
If his father is a tosser, well, then that is what his father is. Many children have substandard fathers and become well-adjusted adults.
If his father is a tosser and yet his mother stayed with him despite that, then your son will always feel angry that his mother didn't stand up for herself and also didn't put him first and get him away from there.
He will love you and he will learn to be a tosser to women that he loves.
He will love you and he will love his father and he also will spend his young life trying to manage the relationship between you and his father, and he will feel RESPONSIBLE and GUILTY for your bad relationship. He will become that sad, sad thing, a Very Well Behaved Polite Little Boy (who hides his anxiety and rage), because he will believe it is his job to make things better. He will learn parent management rather than have a proper childhood.
He will have no 'normal', no reference solid base.
What I mean OP, is that there is a lot more to putting your son first than you can see at the moment.