Yes, you are in charge.  Your dad is only 55. He is in your care. If he wants
to live on his own and kill himself with his insane food choices that is his
right.  If you only have decent food in the house than he will have a choice
of eating it or leaving.  If he leaves you won't have to participate in his
suicide.
You are living with an addict. My advice is to treat him as such.  If you can
get him help great.  In the mean time, if he can handle it, you may want to
give him paleo higher fat foods such as avocados.
I don't envy your situation.  The only control you have is what you bring in
to the house. -David

> Ultimately I am in control of the
> food buying, and if I just stopped buying the
> hershey's and velveeta, he'd just have to deal with
> it. I'm not that dictocratic or mean, but I also feel
> like I'm supplying an addict.
>