Do you have power of attorney for him?
TBH even if you do, in my experience the hospital won’t be too fussed provided they’re happy he’s met the criteria for discharge.
Are you able to go in to the hospital? Ask to speak to the ward manager and explain the situation. Keep repeating it’s an unsafe discharge (explain why) and say you refuse to accept responsibility for him if they discharge him and he’ll be back in hospital within days. Be firm, they will need the bed and will try to push him home.
They will probably also threaten to evict him if he doesn’t leave. They can do this, but not immediately, especially if you’ve explained it’s an unsafe discharge.
Ask to speak to the social worker in charge of his discharge too. Explain everything to them.
Whenever you’re speaking to someone, take notes of their name and what’s being said. You will get told different things by everyone. Make it clear you’re writing things down because you know it’s an unsafe discharge and want to be able to call people out when he’s readmitted in a few days.
It’s a really stressful situation but you have to fight for them to keep him until there are appropriate support networks in place for him at home. Has anyone even asked you what the set up at home is?
I’ve just been through this with DMIL (twice) - good luck and stay strong.