I don't know if there is a legal age for this... I think here it would be similar to what Petrochemist described, that a child this age would not be participating in figure drawing art classes (or photography classes) with nude models. A parent posing nude for her preteen son seems to be at least questionable. And it's different than family situations in the home where there may be a certain amount of being undressed or running around in your undies! lol But he's not a little kid either at 11.
I don't think it's possible to figure out the situation based on such little info., and not that it's up to us to judge it. But from the info. given, and from what I know from years of working with kids, this seems like it would be reportable. The reason is that while there could be some reasonable explanation (a possibility that the family could be naturists, or maybe consider this to be acceptable) there's also a possibility that there could be other inappropriate sexual contact happening in this child's home. It seems like it would need to be checked out.
It's probably unlikely that the OP would be able to do much anyway, but I'd think as likely as not this kid will be talking about this and at some point it will come to the attention of adult(s) - who I would think would find it questionable and report it and get the situation checked into.