Hmm, a bit of a pickle. Here's why: woody plants need to be able to move in the wind so to strengthen their trunk and branches. Staking usually prevents this, with damage as a result once the stake is removed.
However, if the wood isn't completely lignified and can be bend, you're still able to decide the direction of growth and shape. However, this usually is done by staking, which as I said above is not advised long term.
So here's something that MIGHT work, taken from the bonsai community. If the part of the central leader that's bent is still flexible (and not too thick!), you might be able to use heavy duty wiring to straighten it out. However, this will need to be cut, loosened or removed periodically as not to inhibit secondary growth (girth) and possibly girdle it.
Or learn to live with it. I myself have planted a larger hornbeam specimen this year, and when you look at it closely, from the trunk up to the tip has a light S shape. I don't really mind at this point.
