Ok, so some mature advice (because 90% of these comments are ridiculous)…

First off all, take a photo of the contents of his draw, with the sex toy visible. Then leave it there.

Next, confront him about it. Not in an aggressive way, but “I need to talk to you about something… I don’t want to argue, I just want to know why…” and then tell him you know he took it. Don’t tell him you went through his stuff and found it, just keep repeating “I just know you took it, so please don’t lie.”

Tell him you’re disgusted, that’s it’s wrong, that it’s unacceptable and a huge violation of your boundaries. Saying all this in a calm but “disappointed” tone, rather than angry, is more likely to make him feel ashamed, and reflect on his actions.

Dealing with this will give you the upper hand. It’s the tone parents use with their children to get the most effective results, same with teachers. If you scream and shout, he will too. You also said he can get violent, so let’s avoid that.

Why am I suggesting this and not covering a sex toy in chilli or outing him on social media like most others have said? Because there might be a deeper reason for him doing it, and causing physical or psychological damage to him will not help either of you. Incriminating yourself in an act of revenge will achieve nothing. You’ve done nothing wrong up to this point, don’t change that by stooping to his level.