Usually it's just weak gums, I sometimes bleed when I brush my teeth because they arent strong enough but it just varies from person to person. taking a hard brush to a not that strong gum would cause trouble. I guess the key is to find out how to strengthen gums and if you find out, let me know xD
I'm PRETTY sure, any sort of gum should not bleed unless that person sucks at keeping up good oral hygiene D:
Anyways he says he bleeds with the lightest brushing. He used to be a healthy kid.