Well, the "why should I" is that in my opinion your ex's salary becomes as irrelevant to your options as, say, your next door neighbour's salary.
If you've stopped work or changed work to support him and both your son, then that absolutely should be reflected in your settlement.
And he must pay for his son.
But beyond that - he just becomes some guy that you have a different income from.
It does sound like you'd be happier moving out, and I'm sure your son would be fine with it.
I don't think chipping away at your savings (after sale) is a good idea though.
But whether you sell up and rent, or look to buy, I think you need to accept that you're going to a smaller / less "desirable" property.
I might in your position rent temporarily if I was planning to go back to work (or your home working business is projected to grow enough for mortgage) but I would go for a smaller cheaper house, not try to maintain the same.
With the financial control thing though - remember you have a choice about feeling controlled.
If he says he's paying for everything, and you just think "well, you're not, it's a fair settlement so nob off you cock" then he simply gets no control from it.
In my case, ex couldn't afford to buy me out, but he could afford to take on mortgage. He also has a prospect of buying me out later. I could have taken on mortgage but other outgoings too high. And I'd never be able to buy him out. So I proposed he stay put (for our daughter's continuity) take over the mortgage and I put a legal charge on for my equity.
I don't think for a moment that he feels controlled by me having an interest in a third of the house!
I bought a smaller place - and I'm really happy here.
Having a job gave me mortgage options. If I were you I'd get back to work ASAP once settlement is sorted. Once you've got a mortgage you can concentrate on your business instead.
But I think you need to get out of the mindset that thinks about his riverside apartment. Get a fair settlement, but then you have to let it go that he earns more than you.