My boyfriend thinks I’m cheating on him but doesn’t know that I know.

I 25 f started receiving random texts from men sending dick pics to my number. I brushed off the first one thinking it was a bot. But then I got two other numbers doing the same thing. A sweet samaritan from a group in Reddit texted my number and told me what was really going on. He said that my man 26 m was asking other men to send BBC pics to my phone to see if I would cheat. He sent me screenshots as well. I mentioned to my boyfriend that I was getting more strange texts. He wasn’t too happy when I sent him a screenshot of someone sending a dick pic. Then I told him I am thinking of changing my phone number if this doesn’t stop. His voice got higher saying “I don’t know why this could be happening to you”. I don’t know what to do. I love him dearly and despite all the stuff we’ve been through I still want to be with him. I rarely talk to my friends let alone see them. I spend 90% with him or my 5 year old daughter. Should I tell him that I know it was him who’s behind all this? Or should I let it go?