Okay dude. I think as a Victim Of This Specific Issue, it might be maybe helpful for you to get a little insight from someone who's sitting closer to his side of the spectrum. I don't know if any of this will help you out but i actually just faced a minor crisis regarding this exact problem so if the knowledge i gained from that little fiasco can help you out, then cool, it wasn't totally negative!
Basically: i am super totally paranoid of being replaced. Like, it literally haunts my dreams, just like it reportedly does to your boyfriend. I'm basically constantly on edge because she could just totally remove me at ANY SECOND and jesus it's so stressful. I think it comes in part with low self-esteem because JESUS CHRIST I'M STUCK A DORK WHY THE HELL IS SHE WITH ME clearly she's going to leave me any second now. That's something i struggled with for... quite a long time, a little over a year, and i told her about it frequently (just as your boyfriend seems to do), and she'd reassure me. It didn't really put me at ease that much but at least when i was feeling down about it i could go, oh, but Courtney said that she actually won't leave me because i'm not a piece of shit! I don't personally believe that but at least i know she does, so it really does calm me down when i'm feeling off. I know that simple reassurance might not seem like a lot, but i swear it goes a long way... even if it doesn't seem like it.
The thing that really stopped my worrying on this, though, is something that happened super recently. Basically, Courtney got asked out by someone else, and the only reason she didn't do it is because she was told she wasn't allowed! (Sometimes being a teenager and living with parents actually works out for me in a good way, apparently.) That was really hard on me, because after all the promising she'd done about not replacing me, she'd done and effectively done it. Sure, she wasn't able to follow through, but she would have if she'd been able, and that's what counted. So we had a good long talk, a few ugly things came out of it, but she said this: "I know it hurt you and i regret it, but i just want it to be clear that no matter what path i take, you're at the end of it." We ended with a compromise that neither of us will see anyone until we're allowed to be together again... I can't recommend that for you, as it's a bit limiting and controlling and doesn't really apply in your case, but a more tailored version might be assuring your boyfriend that no matter what happens, he's still number one! No matter what you say, he's always going to have doubts, but if he can go back to something you've said in the past that can calm him down, that's really all you can do and i promise it really does make things a ton better. Just be there for him, and remind him that you're not some floozy off the streets! You're a classy lady! You aren't just going to leave him because you've grown tired of the same old toy and want something new. As your relationship progresses, i'm sure he'll figure these things out for himself, but for the time being, a little reassurance goes a long way. Scout's honour.
Basically: i am super totally paranoid of being replaced. Like, it literally haunts my dreams, just like it reportedly does to your boyfriend. I'm basically constantly on edge because she could just totally remove me at ANY SECOND and jesus it's so stressful. I think it comes in part with low self-esteem because JESUS CHRIST I'M STUCK A DORK WHY THE HELL IS SHE WITH ME clearly she's going to leave me any second now. That's something i struggled with for... quite a long time, a little over a year, and i told her about it frequently (just as your boyfriend seems to do), and she'd reassure me. It didn't really put me at ease that much but at least when i was feeling down about it i could go, oh, but Courtney said that she actually won't leave me because i'm not a piece of shit! I don't personally believe that but at least i know she does, so it really does calm me down when i'm feeling off. I know that simple reassurance might not seem like a lot, but i swear it goes a long way... even if it doesn't seem like it.
The thing that really stopped my worrying on this, though, is something that happened super recently. Basically, Courtney got asked out by someone else, and the only reason she didn't do it is because she was told she wasn't allowed! (Sometimes being a teenager and living with parents actually works out for me in a good way, apparently.) That was really hard on me, because after all the promising she'd done about not replacing me, she'd done and effectively done it. Sure, she wasn't able to follow through, but she would have if she'd been able, and that's what counted. So we had a good long talk, a few ugly things came out of it, but she said this: "I know it hurt you and i regret it, but i just want it to be clear that no matter what path i take, you're at the end of it." We ended with a compromise that neither of us will see anyone until we're allowed to be together again... I can't recommend that for you, as it's a bit limiting and controlling and doesn't really apply in your case, but a more tailored version might be assuring your boyfriend that no matter what happens, he's still number one! No matter what you say, he's always going to have doubts, but if he can go back to something you've said in the past that can calm him down, that's really all you can do and i promise it really does make things a ton better. Just be there for him, and remind him that you're not some floozy off the streets! You're a classy lady! You aren't just going to leave him because you've grown tired of the same old toy and want something new. As your relationship progresses, i'm sure he'll figure these things out for himself, but for the time being, a little reassurance goes a long way. Scout's honour.