Heartbreaking to hear about your DS situation, [hug] 
However, having had my share of desperate times with two autistic DC, I would say that you need to stabilise your whole family situation. The idea that you are to embark on home ed against your instinct, in financially unstable set up, partner not able to work, selling the house, is not going to help your DS at all, your MH is going to suffer and your ability to carry the whole family with it. I never considered home education for my sanity. If you don't feel you can embrace it, it is probably a bad idea. If you can't cope delivering home education, things will deteriorate rather than get better. I think you need serious support and advice.
I had to reconsider all my 'red lines'. The ideal situation just does not exist, you need to play with the boundaries (those red ones).
As a speculative long shot, if there is no school currently that can meet his needs, try to 'create' such a school place. I don't necessarily mean for form a free school, although it could be an option. I mean talk to various potentially partially suitable schools and figure out what extras they need to meet your DS needs. Then fight to get that. I feel somehow that you need a sympathetic mainstream school that could build the suitable support around your DS.
My DS went to grammar school and it was a success. They didn't exactly wait for us with open arms, but I talked to them and they built nice provisions around DS, so it worked very well. Your DS being gifted is a huge asset, he is lucky. The worst situation is for DC on the spectrum who are average - nobody cares at all, nobody has a stake in pulling up their attainment, they just cascade down to the bottom set and are left there. In your case they can't discard your DS. Find people who want to grab that. In my experience and from what I read on MN a few times, selective schools are actually more accommodating to ASD. Because once the selection test is passed, they accept your DS is bright, it is in their values to nurture bright kids, for them he is 'deserving', and all they need to do is to put in place provisions that work. Grammars have fewer DC with EHCPs, so they can and do give them more attention. Often those school are quite competent, so they can turn their competence to meeting your your DS needs, a new challenge. In my experience, grammars actually want to make it work. You have the problem of explosive behaviour though, and I don't have any experience with that. I suspect private mainstream schools would opt out.
You also need to create a suitable culture at school, accommodating, anti bullying, pro social support, where patient peers would act inclusively. My DS's school organised buddies and 'friends' for him. So he felt he had people to talk to.
My understanding is that with anxiety and challenging behaviours DC cascade in quite difficult specialist placements with eye watering fees. So your LA should have an incentive to make it work.