How does reading ahead make him immature?
What if he's played/run this adventure before and he just knows it from experience? Is one immature because they've played D&D sometime in the last 40 years?
What if he read up on them ages ago and has a good memory, maybe he really liked this adventure and was really looking forward to playing it some day, is he immature because his biology affords him a good recollection of past events?
I played Castle Ravenloft recently, I read ahead, but didn't use out-of-game knowledge to ruin the game, but I was somewhat unimpressed with how my DM handled the game, I felt like a lot of what seemed interesting the campaign guide was left out. If I had not read ahead, I'd have no idea why people gave two coppers about Ravenloft, it was that much of a letdown.
I understand taking offense to reading up...and then spoiling the game. But doing the former does not imply the latter. Though the player sounds like a problem in a variety of other ways, which I think this would add up to the straw that broke the camel's back.