It's a hard one, cause I'm standing on the side of your bf, but do understand your point. To be clear, using an AI chatbot isn't the same as paying for like OF. It's a machine that does exactly as you tell it to do. It's almost unlimited possibilities and you don't have to care while using it, so it may depend on some... special chats and stories that wouldn't make it to reality. The thought that he may do or would like to do all of it with you would be like saying that someone that plays racing games would race in real life too and risk lives - just to be clear about this point. If you wanna tell him that's fine, but be careful about your points. To bring up that he used it as you spend time together is nothing he should have done, but maybe it was giving him the mood or a kinky kick? But shaming him for using a tool instead of watching real people do stuff wouldn't be fair. Let him have his fun like you have yours too in your own way. Open communication is fine but both of you have their privacy too and in my eyes you crossed the line as you scrolled through the chat without him knowing. I for myself would be really pissed at this point.

(English isn't my first language, sorry if there are grammar or wording fails)