According to Alan Dean Foster's TFA novelization, it was Ben.
A request that wouldn’t go away, he knew. It would never go away. He made up his mind. Instead of retreating, he advanced. Instead of running for safety, he took up the challenge. There was no real choice, he told himself as he advanced to the edge of the walkway. And called out.
It echoed across the gap, reverberated through the vast open space below.
On the far side, a tall figure turned and retraced his last few steps.
“Han Solo.” Kylo Ren stared across at the older man. “I’ve been waiting for this day for a long time.”
“Take off that mask.” Han’s tone was a mix of command and empathy. “You don’t need it. Not here. Not with me.”
“What do you think you’ll see if I do take it off?”
Han moved forward slightly. “The face of my son.”
Please note that his last name remains (so far) a mystery, since Leia retained her original surname of Organa in the new Disney canon, so Ben could have been either Solo or Organa.
I couldn't find any details on the surname in the novelization, or the new visual dictionary articles, or Databank.