Our father has dementia and he can’t fast?

He is exempt from fasting due to his medical condition such you described.

Please do some research on a person who cannot fast due to medical reason.

You have to help poor for that manner. Please do some homework, and ask Scholars. 

Fasting is not to be done by those who are seriously ill or are in a condition where fasting would affect their health etc (eg fasting on your menstrual cycle, if diabetic, etc) 

Your father should not be fasting, especially because he cannot remember etc. Please tell him not to try to fast and I’m a bit surprised that you would think that he should be fasting. God does not want to burden us more than we already have.

Fasting is a exempted for people who got severe illness, who are dependant, and who needs to follow medicine on time.

For example, 

1. Patients that need to eat ( pills, food, etc etc)

2. Breastfeeding mother or mother who did Ceasarian

3. People who got severe illness like gastric problem, etc etc

I'm so sorry that you are very ignorant and using terms like "can he get away with it" shows that you are ignorant. What is his crime for simply being ill? and not being able to comprehend his surroundings and clearly not sound of mind which makes him not as aware. Which makes him unfit to fast. Obviously some Muslims users and user Capitalgentleman have told you the correct answer while other users were being very rude and very disgusting. People who are ill, Ar-Rahman has permitted them to be exempted from fasting and charity can be given on their behalf. Ar-Rahman Almighty is very merciful towards His servants and never burdens anyone on the things they can't do. Your poor ill father deserves to be treated with kindness and make sure to have someone pay for charity on his behalf. Allah Almighty never made His religion with difficulty, don't make things much harder for your ill father.

Okay.  Fasting is only for those capable of it.  If not, e.g., because of illness, it is not required.

He should not be fasting since he cannot form intent. Charity can be given on his behalf. 

Those who are in sickness then being excepted from fasting.

Allah has no concern for the mentally deficient.

Omg, get away with what?  You religious folks are unbelievable!!  What abuse, your dad's brain is deteriorating which will soon cause his death, leave him alone with your religious abuse, your rules and pathetic rituals no longer matter, they never did and they are certainly not going to save him from your fairy god daddy. What a joke!