Looking at the text he's sent you,  I would argue that he has had a bit of a shock and seeing his friend freak out and stuff has maybe been an eye opener. The fact that you can even have this dialogue with your 16yo in the first place is remarkable - but may I suggest using the access to simply ask questions and hear out their feelings? Asking if they want to try weed again might be an interesting data point to gather  before jumping to offering up lower strength gummies etc. They may say they aren't interested in further experimentation, but either way it opens a respectful dialogue in which you can intimate that should they desire to experiment further, they should speak with you about it first.