confused

So my boyfriend and I have been dating for almost a year now .. and so I had his phone one night because mine had died and I needed to search something online , while I was on his phone someone messaged him , the number wasn't saved so I got curious and checked it out , what I encountered was messages with another guy and Penis pictures, but my boyfriend is using an alias named Steven Rogers... I'm so confused on wether or not if this is just a sick game that boys do bc they get bored and like to mess around , if this is just some sick joke or if he's being paid to sext kinda like a prostitute ... I dont know what to think or how to go about bringing it up with him ... 😓🙊 ADVICE PLS HELP.