No, the average twelve-year-old wouldn't require such a degree of supervision. However, average or not, you're faced with managing the behavior. It's reasonable to expect that he shower, that he be truthful, that he not be sneaking, that he not hurt the younger children (or anyone, for that matter). You really are his surrogate parents, so it's important to have a plan re: discipline, including consequences for misbehavior. This can sometimes be awkward for an older sibling to do, but it's important. Otherwise you lack some leverage to influence his behavior. Do you punish him for violating the family rules or expectations? If so, how? What do you think your options are?
I don't really know my options. When he is punished or repremanded he freaks out. He yells, argues and/or runs away. He will come back just fine like nothing ever happened. Meanwhile, we are all worried about him. I don't want to be too hard on him because I think the boy has issues that I can't solve in one summer. I just don't want to mean or let him get away with too much. He doens't know when certain behaviors and actions are appropriate and I don't know exactly how to tell him. Any help would be greatly apprecitated.
You are certainly correct in thinking that any baseline conditons he displays will likely not be subject to change over the course of the summer. Having said that, he is in your care for the summer, and it's fair to expect reasonable behavior from him. Perhaps you and your husband can sit down with him, review what's happended thus far, lay out your expectations, and establish some standard consequence for violation of the rules. The implication is that he needs some help back home, and anything you can do to further that would be useful.
For openers, I think it CRITICAL that you not over-react to his "frieking out" or yelling or running away. This sounds manipulative to me and if he believes it gets a rise from you, he will use it. You are surrogate parent- in loco parentis- and if I were in your position (which I am not) I would not hesitate a single bit to spank him in his underpants. Or, at least place him over your knee in his underpants and let him know your expectations. This may be a sufficient wake-up call without having to actually spank him. Just going over the knee can get a powerful message across. However, if he gives you lip while over your knee or attempts to flee, I'd redden his buttocks. And, I'd let him know that as long as his behavior continues to be childish, he is asking to be spanked like a child. I've known parents who only had to do it once- and the lad got the message loud and clear. He is testing you!!! Pass the test! If his behavior is severe enough, I wouldn't hesitate to drop his underpants and spank bare bottom. That may sound extreme for a 12-year-old (usually the cut-off point for me is age ten) but enough is enough, don't you think? Just be careful not to going beyond red.