Thanks.. that really made me feel better. It hasn't been brought up again, and things have been ok on bus for past few days so hopefully its forgotten. I can't even remember how I had replied to that when he said I must have told someone, so maybe I was jumping to conclusion. He might have figured it out but maybe I didn't say.. lots of good advice here, thank you!
chances are it was all you (and that's OK ) but you can (if it comes up again) remind him that other too can say things about his behavior - other children can be frightened and tell their teacher or parents, etc. about it - some might not know it's play and can interpret it totally differently!