MIL was/is diabetic - we never managed to stop the only sweet things to eat for her until she went into care where they have been able to get her to eat 'normal' meals.
At home we tried to limit the things she did eat - fruit, popcorn biscuits and cakes - by not taking her shopping so regularly- there were 'sensible things' to eat in the house like @canary suggested but she didn't eat them so would have nothing!
It is a very difficult situation to manage - even the GP said just lets her eat what she wants and never mind the diabetes!
I wish you well in trying to manage your Dad's eating.
