In my experience, B&W tweeters seem to "pop" more often than those found in other reputable brands. As such, your tweeter probably is "kaput". As you surmise, it probably was your son but no way to know for sure. Have mercy on him though as i remember being in a similar situation when my Dad had his system and i used to use it : )
With most of the earlier B&W's, one could simply swap diaphragms and be done with it. This is not difficult so long as you aren't a complete "fumble fingers" type of person. If you contact B&W and the speakers are still under warranty, they might even cover this.
My suggestion is to replace both tweeters at the same time. This will maintain a balance between speakers. Should you ever blow another tweeter, you'll have a spare that is already "broken in" similar to other that was left operational. Sean
PS... Sounds like your son needs a system of his own. My first "system" was probably the best gift i ever received. Then again, you can see where that led, so be careful... : )