I don't know how it works in this school system, but in ours, all parents have to do is send in a note explaining their child's absence and it will be excused. I have never heard of a child being penalized for absences due to illness, so I have a hard time believing that the school is punishing this kid simply because he's frequently sick.
You are probably right, concerning a single day's absence. Many schools require a note from a Dr if a child is out 3 or more days in a row.
I guess it is possible that the child was sick and mom didn't get a note. I doubt it though.