He is a doubting Thomas who needs to see the evidence first.
doubting Thomas
idiomOne who requires proof before believing.
Origin
disputed originOriginates from the story of the Apostle Thomas in the Gospel of John, where he refuses to believe that Jesus has risen from the dead unless he can see and touch the wounds of Jesus himself.
I told my boss the project would succeed, but he is a doubting Thomas and wants to see the data.
Despite the expert's convincing presentation, the doubting Thomas in the back of the room refused to accept the findings without further verification.