How to get over his past?

I know this sounds immature and childish but I cannot get over how many girls he has slept with. I dont know a number but I know he definitely got around. We’ve been dating for four months and he’s amazing. He doesnt seem like the “fuckboy” type at all but I know he was. I’ve only slept with one other person who was my first love and long term relationship. I have high anxiety and the thought of him with all these other girls literally consumes me. How do I get over it. I keep telling myself, hes with me, he doesnt think about them and his past is not my business. But it lingers in the back of my mind ALL the time. I literally cant stop thinking about it. (I assure you that I know I sound like an insecure child, but i just need a way to stop thinking so much) Advice??