I just came across this thread and thought I'd update. So much for this being a "burning out" passion - 8 months later and he's still going strong and still improving rapidly. He now plays at the level of a skilled adult. He has recruited several adult family members to play with him (which is kind of a fun thing to be able to do with long-distance family members!), and he is a really tough opponent. I now consider myself lucky if I can win half of my games with him. I looked at his game-collected stats the other day, and he has now beaten more than 130 unique opponents and played over 7,000 unique words! It's been quite amazing to see how rapidly his skills have grown.
In terms of my original concern about screen time, we've settled into some routines around when he is allowed to play that allow us to be comfortable that he's not spending too much time on it.
