By who?
Obama cannot prove his eligibility now or he refuses to do so.
Or doesn't have to. Any more than Bush or Reagan or Clinton or any other president has had to.
This happened before he was president and was a senator.
No, most of these suits have been filed after he became president and challenge his eligibility for the office and not his eligibility to run. The U.S. Attorney's office handles the defense. Those suits that were filed before the inauguration were defended by private counsel. But in the overwhelming majority of the cases, Obama has not had to produce a defense at all. In the 62 or so Birther cases that have been filed, only 10 required a lawyer appear in Obama's defense. Three of those were before inauguration day so a private attorney was involved and seven were handled by the U.S. Attorney's office.
Wrong! Leo, Berg and Martin all filed before. Obama was visiting Grandma when Martin was in court in Hawaii.