Please feel free to PM me if you want to talk privately. I'll reply to this message here though in case it gives anyone else any food for thought!
I'm sorry your DS is struggling. Sensory issues are a big part of my DD's life (and therefore my life too!) and I can't imagine how difficult she'd have found a classroom.
You don't need any specific teaching knowledge to home educate well (in fact, the home educators I know who used to be teachers say that they have to disregard everything they did in schools when educating their own children!).
In terms of what to teach, the law is clear about home ed. Every child must learn numeracy and literacy and have opportunities for socialising and physical activity, but how you do this is up to you. The beauty of home ed for autistic children in my experience is following their special interests. For example my DD was absolutely obsessed with the Vikings for ages so we made it a huge topic where she learned history but also geography, science, handicrafts and art, and we themed a lot of her literacy and a little of her maths around it.
You can follow the NC if you want to, and the info is all online and also in many workbooks you could buy. We only use the NC for maths, but it's very easy to do as I can buy workbooks that take us through every step of the content.
You don't say whether your DS is working at a y3 level but another beautiful thing about home ed is that it doesn't matter. If he's actually working at y3 level in most things, but his spelling and handwriting are y1 but his maths is y5 for example, that's fine! You can meet him where he is and that's a much easier place to make meaningful progress from.
I strongly recommend the website Twinkl (your son's school probably gets a lot of their work from it!). A couple of years ago they used to give you a free resource every week for signing up to their newsletter but they might have stopped it now. They have quite a few free resources and samples you can download and they have everything you could possibly need to home educate at primary level.
I also recommend the Facebook group called PopArt Home Education for anyone who is home educating - they run fantastic discounts for educational resources. We use PopArt to get a big discount on the White Rose maths online platform, which is used by most primary schools. It costs me about £15 a year to have access to all the same maths a school would have access to.
If you have any questions about home ed, I'm very happy to talk via PM or in this thread. Good luck to you and your DS whatever you decide - parenting an autistic child isn't easy and watching them struggle is so heartbreaking and I wish you all the best.