I'm scared

Well, I'm so in love. And me and my bf have had sex but not in a while. I wasn't his first but he was mine. He said that he lost interest when he started having sex with his ex and then broke up with her like 2 weeks later and says he wants to be with me for forever and doesn't want to risk losing interest. Is he being so sweet or making an excuse cause he doesn't want to have sex with me?! I just don't want to lose him...