How do we get in control of my father's financial situation?

jeannegibbs is right. I have a few more suggestions. Is there someone in the family he would trust (a sibling or a parent)? Would he trust a family friend (hopefully someone honest with good bookkeeping skills)? Is there a spiritual leader whose guidance he would follow? Would he trust his doctor to pick someone? jeannegibbs is also right about the paranoia. Can his doctor change one of his medications to one which lessens paranoia? Good luck and blessings on you.

Do you have Power of Attorney? Is your father willing for you to have authority to act on his behalf?

The sooner you can get control, the smaller the mess you'll have to clean up. I hope that Dad will feel relief that he doesn't have to struggle with this anymore. (Heck I don't have dementia and I'm not retired but I'd LOVE for one of my kids to manage my finances for me. I get no volunteers, though.) Some paranoia is not unusual in dementia, so don't take it personally if he resists and doesn't seem to trust you. Just keep at it and try to catch him at a "good" time.