It looks like you have made a very earnest effort to see if this boy is in danger, and you just don't know.
A few years ago when my son was in first grade I was reported to family services because he was out a lot due to sickness, and he was having problems in school. At first I was outraged, but then, after I talked with the family services, I was quite glad that they had a policy in place. Once I explained things, and they could see that he was not abused or neglected things were fine. It was uncomfortable for me, but I got through it.
As a parent, I sometimes feel that society puts too much emphasis on "the village" taking part in raising a child instead of the kid's family. HOWEVER, if you just don't know...if it really could be either way, I encourage you to report it. Do it as Big Tex said, just like you did here. Make it clear to protective services that you just don't know...it could be either way. But better that the child's parents be innocent and get ticked off at you, then the boy or other children in the house being beaten.
Maybe the mother is telling the truth. If he has been acting odd, I wonder if his injury is more severe than she realizes. Someone needs to report this. Just be clear that it could go either way. In any event, even if this was just an accident this kid obviously needs help dealing with school. It sounds like his parents have not been able to help him, so many some on in protective or family services can assist them with that.
This is such a hard call to make. But keep this in mind: A few years ago a little girl named Kassidy Bortner died as a result of her mom's boyfriend constantly beating on her. The people that babysat for her and her mother never did anything about it, and she died. I think she was barely 2 years old. The babysitter was an elderly man, and he will never forgive himself. There are a lot of Kassidy Bortners out there...god help anyone of us that knows one and does not have the courage to do something.