That's just his basic behavior. You tell him he's not smart, he needs to get official (as much as twitter can be considered official, in his presidency) and say that he's the smartest of all time. A dangerous dictator warns he's got a nuclear button on his desk, he needs to respond saying his nuclear button is bigger and more dangerous. Trump is basically a 8 years old kid, not even a well mannered one. That's why it is a very, very bad idea to let him stay in a position of power. I've followed his campaign and his election from afar, seeing how I live in Italy, but I still could see this coming very clearly, because his behavior never really changed from the moment he started promising "a very big wall that Mexico will pay for".