My boyfriend and I met through an application, but I had sex with?
I think that being open and honest about your past and sex life is a great thing to be able to do in a relationship because it shows a level of trust that some never reach. With that said, I dont see why you would need to tell him this. Is it going to help your relationship in any way? Does he have a kink where he wants you to sleep with other guys? I just dont see what you would accomplish by letting him know that you slept with some other guy a week before you met. It was your personal life before him and it doesnt really impact him in any way.