welcome to Healthcare Magic.
From the history it appear that your father is suffering from psychotic illness. It may be schizophrenia
or Delusional disorder
. In psychotic illness patient do not have insight and they do not consider them-self ill even after lots of confrontation and conversation. Due to lack of insight they do not accept medicine.
In such cases medicine can be given in food after psychiatrist
advise. Your psychiatrist already prescribed liquid preparation which can be mixed in food easily.
Wish you all the best.
Dr Ashish Kumar Mittal