Do you have his beak trimmed/filed? If so was this done recently? He may have also just chipped his beak on something.
There are a lot of nerve endings in some areas of the beak, so a tiny chip could be very painful (much like human teeth, though the materials involved are a bit different.) If that's what it is those nerve endings will die off soon enough and he'll be back to normal. But of course if thats not what it is...
We are often quick to send people of to the vet - and in any medical situation that is the best advice one can give. But in reality none of us bring our birds in every time any little oddity comes up. Perhaps just a phone call to the vet would be appropriate - I suspect they will give some simple advice and say not to worry unless it gets worse. But on the off chance it could be something else they might recommend to bring him in. A rush trip into the vet may be overkill, but a quick phone call in the morning for the peace of mind is easy enough.
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Unless of course we have a vet on the forums here. I know we have one in vet school, but she hasn't been on in quite some time.