What do I do if the dementia patient is my boyfriend's father and he has been coming on to me with sexual advancements?

Your boyfriend needs to know, so he will understand why you never want to be alone with his dad. But your boyfriend also needs to know that this is not unusual in dementia. He shouldn't rush off and berate his dad or disown own him, etc. Dad's brain is broken, and his social filters are weak or gone.

Without dementia, a father might think, "Wow. Son really found himself a hot girlfriend. If I were a few years younger I'd like a woman like that." He'd think it. But he wouldn't say it and he certainly wouldn't act on it. He "knows better." But knowing what is appropriate in various social situation ("social filter") is often one of the casualties of dementia.

As others have suggested, this behavior should be reported to Dad's doctor. There may be something that could help.

Is your boyfriend living with his father? Have you talked about this with your boyfriend? Why isn't his father in an assisted living or in a nursing home?

There may be medications that will help boyfriend's dad with dementia. Where is boyfriend living? This is not all that uncommon for men with dementia.

Back out of the caregiving role.
Are you living there? Are you employed? Do you have your own resources?
Why would you decide to be the caregiver, what is your rationale?

Where are your parents, what are their needs?

Tell your boyfriend of the problem then don't visit his father.