He's...worried about the differences between him and the John you know. That while he might be moirail material in the future, what if he isn't now? I explained to him that nothing is stopping him from growing into that, and that likely you would know well enough if the difference is really significant or not, but he's still unsure. So, as I suggested to him, I'm going to suggest to you that perhaps the two of you should try and wait before becoming Moirails so soon, and spend a little more time together. That way you can discern if there are any differences, and he can decide if he's really up to holding such a relationship. He's worried he won't be adequate enough.

...which, sadly, is a worry of his with a lot of things I'm coming to find. But it simply needs time for him to adjust.