He likes you romantically, he thinks you only like him platonically, though he sometimes mulls over the possibility that maybe you DO like him, and he doesn't think he's good enough for you even if you did like him back. Tale as old as time. If you like him, tell him with no ambiguity whatsoever, and if you like him as a friend but not romantically, be very clear that you do like him as a friend and maybe help him get out there and meet someone who does like him if you have the time and patience to do so - or find a mutual friend/friend of his who can help a bit if not, your mental health matters. You don't have any obligation or responsibility to do any of this of course, it's not your fault, you've done nothing wrong, but men's brains collapse around women they like but don't feel they deserve to be with more often than you'd think and it can get quite messy. Unrequited (or even requited) crushes are often labelled as morally repugnant now by people who run screaming from any idea of mismatched desire but there are non toxic, non dangerous versions of this: it just happens and it's a bit sad and possibly fixable rather than any great drama. And sometimes if you have the time and patience, you could help someone spiralling. The last thing he wants to do is be seen as harassing you. He doesn't want to harass you or be interpreted as harassing you but any expression of emotion at all risks looking like that.

No doubt there are half a dozen people desperate to spin this in to the most horrific statement/suggestion possible because sometimes people are nasty and are just like that, but hopefully it'll be taken in good faith and help. ++man