Rhonda, if you think that sounds angry you should see me in full PMT mode. Not a pretty sight I can assure you. I usually deal with all utility companies, banks, etc at this time of the month as I find them much more co-operative for some reason 
Every boy is different. Some are very immature and some are very mature. We don't know this boy so all of this is speculation.
I have a 9yr son myself who is on the immature end of the spectrum. However in my primary school years I once came across two 10yr olds who were re-enacting homosexual sex. They were even making 'orgasm' faces. I'd like to say that they didn't know what they were doing but they did.
As we don't know this boy it's rather pointless to speculate on how sexually mature he may or may not be. Having an morning erection means nothing. However it is inappropriate to put another child in there as it can make him feel very awkward. This isn't his sister after all or any relation of his whatsoever. I wonder how his parents would feel about it?
I think every parent is paranoid about abuse and it's interesting to note that we have a guest speaker, Diane Abbot, on the topic of pornography and children. It's a sad fact that studies reveal that many young boys have seen pornographic content online, that more girls are being sexually bullied and that porn is slowly seeping into everyday life with our children exposed to more and more inappropriate content.
It's no wonder we are paranoid. Those concerns are typical and normal and it is possible to voice them without having people call for social services to be involved. There is a thing called common sense which can be used.
Someone (probably not you Rhonda) mentioned that the OP was seeking professional advice and that's a sad state of affairs when the professionals have to mediate over every disagreement between separated parents. Not that it's all her fault but I do wonder if she tried every avenue before calling them? The father could be forced to communicate if his daughter was stopped from sleeping overnight. I'm sure he would do the same if the boot were on the other foot.
Good luck OP. Be decisive, use some common sense and get it sorted before it drags on and on.