I'm thinking my gaming addict nephew stopped taking showers so we made up a nasty medical disease to get him wash himself
Why do you think he is a gaming addict?
I've seen him playing those games one too many times. It's very simple.
It is difficult to have a family member go through some type of addiction so it is understandable to do things to get them better. It would be better to handle the addiction in a different way in the future, you can't keep lying to them.
I wish he knew that I was lying to him.
It may seem unorthodox, but your nephew has an issue and it might have been the only way to help him snap out of it. You are doing well in trying to take care of him, and you shouldn't beat yourself up too much on this white lie since he really needed it for his health.