Thanks for reaching out on behalf of someone. I don’t have any information specific to housing in Birmingham, but I would start with your local authority in that respect. Housing provision in different local authorities can vary, and it also depends how far into the future you’re looking - what’s available now, might not be available in five or ten years.
As a more general starting point, I really like this booklet from the Foundation of People with Learning Disabilities (I’m assuming that because you’ve come to Mencap, the son who has autism also has a learning disability? Please ignore this advice if he doesn’t!). The booklet, called ‘thinking ahead’ covers several aspects of future planning, from housing, relationships, finances, wills & trusts etc and I find it quite a friendly and accessible format - it can be looked at by the Mum and Son (and professionals if wanted) together. You can print it out (or order a copy) and make notes on it. It might be reassuring for the Mum, to see different aspects talked about that she may or may not have thought about. It also includes comments from other parents, so it may help her to feel less isolated and scared about planning for the future (if she feels this way).
Here’s the link:
Hope this is helpful