My bf keeps turning me down for sex

Im a very sexual person. We’ve been together 2.5 years. The last couple months our sex life has slowed down. We used to have sex every day or every other. Now its once a week. I bring it up but he always says he doesnt feel good or hes not in the mood. He also says our relationship doesnt revolve around sex which I know it doesnt. But i just like sex. It makes me feel great and its so fun with him. Im super worried about why it has slowed down so much. Hes 23 and Im 21. Advice?

Side note: he started working at a preschool and his only coworkers are women and he talks to them and most have boyfriends but it didnt start til he got this job. I dont think hed cheat on me but i cant help but let my mind go there