He always says the wrong thing because he has foot-in-mouth disease.
foot-in-mouth disease
idiomA tendency to make remarks that are embarrassingly wrong or inappropriate.
Origin
disputed originHumorous conflation of foot-and-mouth disease and put one's foot in one's mouth.
I really tried to be polite at the dinner party, but my foot-in-mouth disease made me insult the host's cooking.
Despite his best intentions to remain diplomatic during the negotiations, his chronic foot-in-mouth disease caused him to offend the delegates with an ill-timed joke.