Bashar Hafez al-Assad is the leader of Syria currently. He became the president of Syria in 2000 so he has been the president for quite a while. Asaad is known for being involved in a civil war where he has attacked his own citizens which includes children by spraying poisonous gas. Other countries including the United States have tried to come to the air of the innocent Syrian people.
Since Asaad has been in office since 2000, it is unlikely that Asaad will either surrender or step down on his own. It would take a war from other countries in order to do that. Therefore, I don’t believe there is anything that will make Asaad step down on his own.
Assad will surrender if the United States military gets directly involved in Syria's civil war. His regime would eventually fall, but this is unlikely to happen. Assad would also surrender if an advanced if an advanced race of space beings descended on Syria and fought on the side of the rebels.
He would never surrender voluntarily. If, however, he is disposed from power one day, then his opponents or other countries might decide to abandon him to be held accountable. Right now, Assad looks set to remain, President of Syria, a country that his family has ruled for forty years. He will likely run for next year's presidential vote.