only one way to find out - question him.
But is there any point dragging him into court if he was only going to repeat 'I heard nothing, I know nothing' to all the questions?
Yes, if there was a way of compelling him to the truth (assuming that isn't the truth), but there really isn't. You'd end up humiliating him on the stand, and progressing the case not at all.
In a lot of ways, an unreliable or untrustworthy witness is worse than no witness at all. That's why they go to town on finding out how reliable each witness is.
It makes me wonder, what would happen if someone was unprepared to take the oath. If they objected to that?