I think these are all great answers so far.
First-off, with it being so relatively inexpensive to get into, and considering your current setup (and not wanting the whole ‘dead son’ thing to happen), I would definitely get him his own hub.
If cross-contamination is an issue (i.e. SmartThings hub1 messing with devices intended for SmartThings hub2), I would simply (ok, simple for me) set up a completely separate Wi-Fi network for him so that neither SmartThings system would ‘see’ any of the stuff on the other one.
I would also consider installing smart outlets in his room which are controlled by your setup, and plugging his stuff (including his own smart outlets) into those. That way, he can play with things all he wants, but you still have the ability to turn it all off if you want.
Of course, there are always going to be kooky things that pop up that you will have to figure out how to deal with, but if you do certain things ‘right’ from the beginning, it will help you out a lot down the road. The first ‘right’ thing you did was to come in here and ask about it. I imagine we’ve only scratched the surface so far in this thread with all of the things you might want to think about when embarking on such an adventure.