Sorry to hear from your dad illness.
Could you tell where his tumor was located on?
Post surgery is not always a smooth period as some people described. It is true, a lot of patients leave the hospital walking and talking, thrilled to know that at least temporarily get rid of the monster. However it is not uncommon that bruises and swelling caused by the surgical intervention bring complications like those you described, especially seizures. Your father is certainly taking some anti-seizure medication, I am sure your doctors should revise it. Patients that never got seizures pre-surgery could have their first one just after.
About your father behavior towards food and his weakness, the main issue for caregivers is to identify those problems that are neurological issues, from those brought by the heavy burden of dealing with the illness. In your father case additionally there are several and severe complications (pneumonia, blood clots), that definitively took their tool over his general health. Anyway, it is important to discover how much of this apathy could be addressed to the cerebral damage caused by tumor and pos surgery effects. It will help you to do the right choices about how to proceed. More than one time I overlook the capacity of the brain damage to alter personality and it cost me tons of energy.
You are absolutely right to temporarily preserve your father from additional treatment for now. It is not easy to get to this decision, but it shows that you are aware of boundaries between treatment side effects and disease itself. I know you probably heard a lot about how important is to improve quality time instead of quantity, and I am aware how foolish it sounds when your father current life is miles away of your image for quality. My advice is to not compare. There are stages where we must reinvent our concept of quality, and start work with new parameters. It is definitively not easy, I am also having a hard time to make it work out, but for now it seems to be the right choice.
I share your frustration to see a fast decline in such short period of time. As I said, I am on the same boat; the difference is that my wife instead of present indifference became extremely agitated and anxious.
Best wishes for you and your family
Mazzo