Indeed, only God knows his heart.
On the passing of a similarly situated man, I extended a word of encouragement to his loved ones - namely, that doubting Thomas was an apostle, too. Don't worry about him, trust God.
Amen! Who hasn't reminded their questioning child of that blessed fact? Even doubt is given by God for His ultimate glory because doubt is never the end, but merely another means God employs to bring His children to Him.
If God has numbered a man among His family, that man will one day, at a time of God's choosing, be shown the wounds of Christ in order to know his salvation has been won for him on the cross by Christ alone.