For next time, let him know that the Scantron doesn't judge his character.
My middle kid left more blank than I think she should have on the SAT. I realize that wrong answers on the SAT deduct a 1/4 point, but I am pretty sure that she narrows down the question to two answers, then refuses to guess. Scored 97% percentile, but still think she should be answering every question.
Eldest, on the other hand, will guess even if she has no idea. It has a lot to do with personality, not sure how much you can change that.