Not looking prepared doesn't mean Cal can't be prepared. Although seeing the helmet does give the young Jedi pause, reflex kicks in as he suddenly senses the surprise from the other.

On his back, his little droid buddy squeals, startled as Cal suddenly jumps back to avoid the charging Mandalorian. Cal's hand snap up as though to counter, which might be odd when he's just moved out of range. Still, Boba would feel it- an invisible force (hah) attempting to knock him back.