Your father himself, or you and your mother are entitled to demand that the hospital discharges him.
You are also entitled to demand a second opinion from the hospital and a reasonable explanation as to why they have stop his heart medicine.
In your place, I would phone your father's GP at once and fill him or her in on the whole situation, so that your father can resume taking his heart medicine upon coming home.
Whether he will get better or not, is impossible to judge from your post.
A lot of old people do stop responding to treatment for pneumonia, which actually is a fairly easy death, if your father has decided that this is his time to die, but he should be given every chance to live a little longer if that is what he wants.
My father, himself a doctor, chose the pnuemonia way out, so I know it was an easy demise for him, although hard for the rest of us.
I am so sorry that you and your mum are going through this,
Bring your father home with carers coming in and his own GP to see him immediately.