The short term solution is to secure your belongings from this kid. Place your Transformers and other valuables into a room, then lock it. But this is only treating the symptom and not the problem, which is whatever issues this kid has that makes him want to steal things.

But yeah, in the meantime I'd definitely recommend locking up your Transformers and other valuables. If he so much as says ONE word in protest, tell him that you're sick of him stealing your things and that because of his actions, you no longer trust him to have access to your collection and other valuables. i.e. this is the consequence of his actions. The sooner kids learn that there are consequences for what they do, the sooner they learn this little thing called responsibility.

You can't control what he does, and you can't control what his carers do (all you can do is ask them to take action) -- but you CAN control what happens to your stuff, in this case, securing them. Keep one key with you (attach it to your car keys), and maybe ask your OzFormer friend that you trust to keep the spare key for you. If he starts eyeing your keys, maybe tell him that if you find your keys missing, the next step will be installing a finger-print scan lock.

I also find it incredibly rude that this child and his carers can live under your roof and disrespect you by allow (through inaction) him to steal from you.