Biggest thing. Even assuming the Dragon somehow suddenly gained all the knowledge and ability to fire the nukes, nukes are designed not to go off so strenuously that blowing them up with other explosive weapons is a standard way to disable them. You could nuke the nuclear silo it was sitting on, and only the nuke you launched would detonate.

Assuming the Dragon survived the nuke we dropped on him [If we decided to], it's just survived the biggest force we can bring to bear on it [Drop a bigger nuke? What for, it'll do nothing but make a slightly larger and prettier explosion.], at this point we just compare how many casaulties it causes to how many people die due to [chosen common cause of death] yearly and go 'You know what, it's not really a problem. If we need we can send some bulls in it's direction to keep it fed.'.