I'm going to take this a step further: allowing a situation in which a person, no matter their age, has no claim of ownership of their own possessions and not only has no recourse if another person takes those possessions just because they want to,* is borderline abuse. So yeah: I would definitely advise LW to give the kid a space to keep his stuff safe.

*Differentiating this from a parent taking them for specific reasons like "you are not getting your schoolwork done because you're constantly checking your phone."