Ex means something that was, but has been disposed of, in a way. It isn't usually a good thing. My former boss- means a boss I used to work for, not necessarily my last boss, just a former boss. My ex-boss, well that tends to mean a parting of ways, not good, with bad feelings. My previous boss, tends to mean my last boss, the one previous to this one. A former president could be any one of the past Presidents. The previous president means the president directly before the current president. I haventvheard of an Ex president except possibly Watergate President Nixon. Notice the negative attachment. Best of luck Esme