You and your siblings might want to encourage him to invest some of the money in stocks for retirement, Certificates of deposit or IRAs, at the bank, and if there is enough, perhaps some (paid off) property of his own. Those make it very difficult to get at money easily, and cut down the chances of impulse buying. I would get property that he can pay in full, so he doesn't run the risk of foreclosure if he has run out of extra money. It depends on just how much money he will be coming into.

Other than that, there really isn't much you can do. He is probably a competent adult, even if he is bipolar. I'm really sorry for those of you that have to sit by and watch this happen, but unless he understands that he impulse buys and is willing to let someone else help him invest and make the financial decisions, nothing is going to change. Sorry.