His practice isn't a foreign concept to myself and others, and while we certainly don't have the same 'dreamscape,' we know enough to understand and educate him on his new found skill - it's similar to ours. Any risks can be controlled with practice, but if you'd also prefer him to try things on his own without any help, well. More power to you.
And he did, actually. You don't understand the larger picture, and quite frankly, I don't care if you or don't. He wasn't in danger. You're upset because your teenager did what teenagers do and not listen to...whatever you are to him.
Your concern isn't misplaced, however, so if you'd like to continue and throw a tantrum, please feel free to do so, if it helps you sleep at night.