I don't think he can prove it without any doubt, that's the point. There's nothing he can actually do to prove it. However, the sentence passed down by the tribunal suggests that they accept his story.
BBC Sport
He was in line for a life ban but because the positive result was "caused by an inadvertent administration of etilefrine", a lighter suspension was given.
"The tribunal rejected a defence of no fault or negligence, but accepted an alternative plea of no significant fault or negligence," said the ITF.
With this evidence I don't think he deresves a career ending punishment.