I believe the first indication of him turning invisible is a group photo that he knows he was in, but in which he does not appear. He starts to turn invisible periodically, maybe not entirely under his control. At first, it seems a bit thrilling that he can turn invisible, to see things other people can't, but he quickly learns that it results in him seeing things he doesn't want to see. One of the cases that comes to mind was a situation of brother-sister incest. There was another one involving adults, but I forget the details... drug dealer maybe? I think there's a scene where, as a teenage boy, he tries to use this new ability to either peep on a girl, or to actually have sexual contact with her, but he flees when he realizes what he's doing is a form of sexual assault. Somewhere near the end of the book, he learns that he has an older male relative (an uncle, perhaps?) with the same ability.
I think I read this in the 1990s. It was marketed as a YA book as I recall it, not terribly thick, maybe around a hundred pages. I want to say the title was something like Blink or Flicker, a reference to his brief periods of invisibility.