At 15', if the trunk has set, you cannot correct the lean. If it is simply a weak trunk (try pulling the trunk the opposite direction, and see if the trunk bends over that way), you may be able to use temporary staking to correct the lean. I have seen that often, where a large willow was planted, and after even a small amount of top growth, has become too heavy in the crown. If you do not correct it, it will set that way, and there will be no way to fix it. Sometimes lightly thinning the crown helps, too.
If the trunk is already rigidly set with a bad bend in it, I'm afraid you can't do much, except train the crown out further on one side, to compensate (kind of centering the tree's weight over the root flare).