I'm really sorry your going through this. I know it's really hard just the emotional part itself, never mind trying to loose weight too.
I've been there, done that, taking care of the elderly. Plus, my husband is a lot like your 90yo dad. He is totally helpless when it comes to feeding himself. I make his Breakfast, lunch & dinner every day. After 40yrs I'm kinda over it.lol
I just had to make up my mind that I needed to loose weight for my health. food will always be around, but I do not have to eat it. Do I want a brownie or do I want to be healthy?? Being that right now it kills me to walk or stand (arthritis in knees), I choose get healthy.
Is it possible to get Meals on Wheels for your dad a least one meal a day?
Your daddy's 90, give him anything he wants, including a healthy child.
Best wishes for you!