Hi Kelly, you're not doing anything wrong.
I really feel for your bf. I know what it's like to be an insecure man, I'm one too. These things come from when we were little boys, growing up, some or other male figure in the boy's life usually acts like an emotional abuser and spirit-breaker of some kind. Hence the useless and insecure feelings. In many cases, it's the dad.
It's really sad. The worst thing is you can tell us a thousand times you love us and we are OK or wonderful, we can never get ourselves to believe it.
I can tell he loves you very much and you are probably the best thing that has happened to him in his life or in a long time. All those childhood fears return in such a case, he is so scared of doing something to screw it up and losing this one person he loves, hence his constant asking.
I think I know exactly where his fears are coming from. He was abused by someone who had an extremely narcissistic and fickle personality, who always believed they were right. The slightest "error", deviation from the iron-clad rules or even simply an alternative viewpoint is heavily criticized and a lot of guilt is laid down on that person for daring to do such a thing. I know this all too well. This abuser is a control-freak and highly insecure himself, and anything not the "norm" is immediately heavily criticized and contested, even if it's right. You learn to walk on eggshells around such a person, never knowing when you are likely to screw up again, all you know is, at some point, you definitely will, and each time it is an extreme guilt-trip and a heavily emotionally tormenting experience.
We never really recover unfortunately, the effects minimize over time, but it requires a lot of things in his life to build his confidence back up. For me, I try teaching and I'm planning to finish my PhD and open my own company one day. If I can achieve those things, I'm hoping it can fix me somewhat.
Maybe encourage him to go for a dream he always wanted to achieve, and then if he sees you support him 100%, it might restore his confidence.
I wish you all the best.