IMO, I would not question him right away. I would get him playing, side tracked, and ask questions then. Also at that time I would let him know that even if the "monster" may look scarey but may just be a normal person and to let adults know if this person tells him anything. I was usually able to get info out of my son when we were in the car since he didn't have to look at me or me at him.
Even if this is a pretend monster, he is doing it for a reason and normally if the kid is out of the area and busy doing something else, he is more likely to tell you details if asked.