My adult son accused my Father of sexual assault and my Father denies it?

My adult son with a frontal lobe head injury said while left alone with my Dad for the weekend was asked by my Dad to see his penis. My Dad then asked to touch it. My son allowed him to touch it before he felt it was wrong. My son has been known to lie, and my Dad has admitted to being bi-sexual. My son swears it happened as strongly as my Dad said it did not. There is a third person in this story that I wonder if I have to blame. She is my parents neighbor and twice the age of my son. She was secretly having sex with my son and has proven to be a liar thief and manipulator. Trying to turn my whole family against each other. I wonder if I should believe my son. Or my Dad that has never done anything like this in the past. Or did the neighbor influence my son into lying again? Of course the easiest thing for me would be to blame the neighbor, but I did see my son jump when he thought that my Dad was going to touch him, and I have heard my Dad make inappropriate comments. But my Dad has also always had a twisted sense of humor. Any suggestion or similar stories would help me a lot. I look forward to you comments. Thank you