You see him three times per week. Rather than be perceived as the know-it-all jerk who tells him to get lights, how about starting with a friendly greeting? There's no rush to get him lit up since he has clearly been doing this for quite a while without getting whacked and a few more trips likely won't get him killed. After you've gotten him to respond to your "good morning" greetings a few times, then you can move on to let him know that he's kind of hard to see. He'll probably own his invisibility and get lights shortly after that.