How's his mom?
Sounds like the other child is not enjoying that homework and it is mandatory for him and he's crabby (best case scenario). It also sounds like he did think it would be "a favor" to someone to keep them from doing their homework...
Hm. I guess having lots of nieces and nephews rolling around the house all the time, I have gotten into a bad habit of telling a child when I think they're being rude, now even kids I am not related to: down at the bus stop, etc.
Usually really freaks 'em out!
(and they'd probably cough up the homework!)
