Re: Is this punishment enough?
At 9 I think he would probably know he shouldnt sign OPs innitials. I don't have a 9yo but it seems that by that age they should know they are "trying to get away with something" by putting their moms signature of approval/acknowledgement on school work.
If he did it with completely innocent motives, I would take it easier on him. But since he probably realizes he did something he shouldn't, I do think the punishment was appropriate.