Hallucinations. What’s the best way to help and do these hallucinations go away?

Your father's doctor needs to be notified immediately and he may be able to prescribe medication. In the meantime however, it's not safe for him to be living alone in his own home. Anyone with dementia to this degree needs to have someone living with them full time or to be placed in a Memory Care Assisted Living home for their own safety. There are a thousand things that can go wrong when a person is hallucinating, needless to say, so I think you need to treat this as an emergency situation, for your father's safety & wellbeing.

You and your brother need to read up on the subject of dementia to see what you and your dad face along this difficult journey. Here is a good article:

https://www.alz.org/media/documents/understanding-alzheimers-dementia-b.pdf

Wishing you the best of luck with a difficult situation.

Some dementias have ongoing hallucinations and my bro was great at describing in detail his night time visitations. He had probable early Lewy's dementia. For them they did come and go and any anxiety brought them on big time. It sounds as though you are not POA and have no power to intervene. At some point he may need guardianship by you or of the state, and placement. If he won't see a doc with you you are somewhat stuck until this is brought to a head, which it will be!

Be sure to contact your dads doctor right away.
Tell your brother to not try to convince dad that the hallucinations aren’t real. That won’t work and will probably upset him more. Not everyone has the same issues and medication helps some more than others. If they don’t go away completely, medication may help them become more manageable.
If you use the search on this forum you will find many posts on ‘hallucinations vascular dementia’. You might find those posts helpful.
Do get your dad tested for a UTI anytime his symptoms accelerate.
Make sure he doesn’t have guns in the home.

Have your brother make a drs appt for your dad. Brother needs to accompany him and explain to the dr his concerns. Antipsychotics may help. Hallucinations need to be addressed. Some hallucinations are harmless, but a visual hallucination can be terribly frightening and may include patients seeing bugs crawling on them or worse. Living alone can be dangerous for him. It may be time to research assisted living facilities.