I don't want to start a argument with him. The tumor has changed his personality and he can be rude and mean. I don't take it personally. I know it's just the tumor talking. I just want to know if I should help him anyway. He wants me put a belt around him and hold him when he steps outside onto a platform. Is that a safe thing to do?
If he is planning on dropping off the platform, to not die of his brain tumor, he is putting you at risk of criminal charges.
It's different for professional caregivers. They've had long experience, and the patient is safer with them.
It's best not to get involved in this with your father, IMO. If you mess up, consequences could be dire. Then he could blame you for that.
Melanoma cancer is likely in other parts of dad's body if it's gone to his brain. Does his doctor recommend in home hospice to keep him relaxed and comfortable? You may want to ask about that.
Best of luck to you
So the simple answer to your question is no, you should not do anything to help your dad if you're not comfortable doing so or if you may get hurt.
It may be time that your dad gets placed in the appropriate facility, where you can just be his advocate and child again, and he can receive the care that he now requires.