The practical issues surrounding divorce always appear insurmountable but rarely are. I remember having a similar dream about Ex dying as it felt like the only way to escape. It is a sign your trust in him has gone.
Your son will feel anxious because there is uncertainty. My ds had a similar reaction and he had counselling as at outlet but what made it better was actually living the new life. You have to go through it, step by step working on the practicalities, and know there will be a good life afterwards.
First thing is to look at finances, what do you think you would have from a 50:50 split and could you buy a house. Even if that means a move, perhaps to be closer to family or your job/schools.
My DC took strength from my reassurance that it would all be ok. I emphasised the positives of the new life and they built confidence once plans started to materialise. I know that sounds so simplistic but it was the reality. If he had to have surgery you would offer reassurance until he got through it, as knew it couldn't be avoided.
For example, they liked to pick rooms in the new much smaller house and they got excited about exploring the new area.
Re the horse, not sure what it costs? It is really ridiculously expensive or could you increase your income to cover it? If you have a long marriage and he is a high earner you may get SM for a period of time so don't assume he can dictate what you will be left with.
Do you have family?