There's no way he could be in the system without medical checkups, and no way a medical checkup could fail to note "hey look he's got unbreakable bloody skin and various other superpowers," so there's no way foster care could fail to know about his abilities, so there's no way potential foster parents wouldn't have been warned. Would some of them still not be able to take it even if they'd thought they could, sure, but it wouldn't be mystifying to his social worker.