Does your partner his stepson treat you like this?
If so, why does he allow it?
So many issues in just one post
I am not sure all the fault relies on the stepson.
Your partner has health issues and only a few thousand pounds left.
How is he planning to support himself for the rest of his life?
Dont tell me you are the plan (free caregiver and bank account)
The house is the inheritance- does stepson acknowledge this?
When your partner passes away, the stepson seems to be the type to create drama
Why did your partner buy him a house?
You are not married - when your partner dies, unless there is a financial agreement or anything legal setup, you are on your own
Are you planning for your retirement and health issues?
What would happen if you cannot take care of your partner anymore?
What if you need care?
How long could you pay the rent?
Summary-
While your partner still has mental faculty, run to a lawyer and make sure you are taken care of if you get hurt while caring for him, passes away, etc.
The stepson here is the least of your issues.