If none of the others are coughing or have any sign of respiratory illness (sneezing, mucus from the nose, bubbly eyes, etc,) then I would guess something like heart or a tumor, something internally wrong. Remember, chickens are expert at hiding that they don't feel well. I'm sorry.
Exact same thing happened to my bantam cochin roo a few months back. Just dropped dead.
I noticed gasping beforehand, like he inhaled a crumble. I checked him and found lice and mites - check yours and see if he has any and check your girls! I don't know if that can kill them but it's not good.