Sorry about your mum, OP - it sounds like you're under a lot of pressure.
I've been where you are in terms of fighting for SEN school places - it can take years, as you unfortunately know, but the good news is that it will work out eventually, and once he is settled in a suitable school this time of all the fighting and emails and phone calls will pale into insignificance.
Have you checked you're getting all the benefits you're entitled to? Carer's allowance, universal credit, DLA for DS, etc?
Do you have a local carer's centre you can contact for support? Mine offers counselling, coaching for carers, zoom meet-ups with other carers (and/or parent-carers) in a similar position, etc, so it's worth seeing what's in your area, or contacting citizens advice or your local library for signposting to local organisations that might be able to help.
Have you considered speaking to IPSEA, Scope, National Autistic Society, etc about school stuff and other support for DS?
Are you in touch with your council's disabled children's team? DS is entitled to a needs assessment and you are entitled to a carer's assessment as his carer, and these assessments could result in you both receiving additional support (such as a support worker coming to help, activities, advocacy support with the school issues, etc).
Does DS have an EHCP already? Do you know how to apply for one? It should be on your council website but there are also other resources available online.