Hi OP,
Are you sure that this setting with support only 9-5 is the right one for your son (and your family)?
You have said that he needs support with everything such as showering, washing and I assume getting something to eat and keeping himself safe. You have said he can't go out alone I think. Wouldn't this mean he requires 24 hour support? Could he leave the building independently if there was a fire?
I have read that you intend to look after him after 5 o clock - to me it seems that you would be massively missing an opportunity here to establish a workable balance for your family.
For context I have worked in a short break service for people with learning disabilities for a decade and I also have a child with autism who won't ever live independently - (although he is only 7).
I am concerned that you might be making a decision about this placement (being the only option almost) based on very limited knowledge. I don't mean that disrespectfully but with the context of the benefits questions it seems like you are feeling your way in the dark.
The adults with learning disabilities I work with, or who move on from our service live either with family or professional carers (like an adult version of foster care) or (when moving on) can live in residential care, supported living or their own property with a team of carers providing 24 hour care (this option is rare and I assume the parents have to fight hard for it). There are many different care settings in the surrounding towns here.
In terms of other support these adults can be attending college (beyond 18), day centre/project type settings such as farms up to every day (with transport contract provided by the local authority) or have one to one support (with the funding for x many hours a week provided by the local authority in the form of a personal budget).
The short break setting I work in people have an allocation of nights to use each year after being assessed e.g. 40 nights each year. These people often still attend their day service while staying with us.
Does your (adult) child have a social worker? If not you need to ask for one and a care needs assessment.
This assessment is to find out what care and support needs your son has but also what care and support needs you have as carers and as a family e.g. to recharge and spend time with your partner or other children and to work.
If you haven't already please try to access this process so you can become more aware of all the possible forms of support options. I think a solution that requires you to care for your child after 5pm until 9am not in your own home sounds very straining on your wellbeing and marriage and not necessarily a good solution after being totally committed to raising your child to adulthood. Where is the freedom for you if this requires you to be working and then provide care out of your home 24 hours a day.
If you thought about what care and support your family and son needs would a service with support ending at 5pm be it?
Edit to add sometimes people also leave our service to go to a residential college.