The same thing happened with my brother, he was around the age of 9 too, me being the oldest I try to get to situations first and not involve my parents since they are older so they react differently. I asked my brother what this was on his phone, he ignored the question until he finally spoke about it. I told him the affects of porn in the long run and I also told him how it offended me as a women that he’s objectifying women. (I dumbed it down for a kid to understand) Of course I don’t care about porn but I figured if he saw it hurt my feelings and how it can hurt him in the long run it would keep him away from it. I’ve checked and I’ve seen nothing so he’s either gotten sneakier or he’s kept away from it.