I think many people think the benefit system is more generous that it really is. However, if someone is unable to work or can’t get a job but is having to live independently from family and has no one else able to house them, they will be eligible for housing benefit and council tax support. If they are living in social rented housing, the level of housing support/benefit will normally be enough to cover the rent unless the property is deemed to be bigger than required - in which case the so called bedroom tax will apply. If they are living in private rented housing, the housing benefit will not normally be enough to cover the rent and there will be a shortfall to find. The age of the person may also affect the level of housing benefit paid.
For a single person, the amount of benefit (excluding housing benefit) isn’t enough to live on and pay for food, utility bills, clothes, shoes and so on and so forth.
We have been in a similar position with a family member in the past and are now having to help another who is struggling with a depressive illness. Been there, done that, so know how little help there is out there. If I can help with any advice or experience, I’m happy to do so. I know what a worry and struggle you’re having to cope with in trying to do your best for your grandson.