I've actually been on the other side of this, more frequently than I care to admit. Although my issues stemmed from depression due to a combination of suckily-borked body chemistry and some horrible stuff that made me not realise how deep I was sinking, the end result is the same: you feel worthless, like no one could ever want to be with you except out of some misguided loyalty that made them feel too bad about leaving you to do what they really want and just walk out.
It's not easy to fight that, for either of ye. Definitely reassure him that you're there because you want to be, but don't be offended if he doesn't completely trust that. If he's decided he's "unlovable" or at least not worthy of your love specifically, there will always (until the situation rights itself) be that wee voice telling him you don't mean it, you're just too good a person to want to hurt him...which, ironically, makes it seem all the harder to 'impose' yourself on someone that good and caring and nice.
Don't want to make it seem all doom and gloom, but chances are there's not a lot you can do to fix it until he is able to and makes the decision to help himself. Try and remind him, not just that you love and care about him, but that he is, in his own right, a good person who has a lot to offer. Encourage him in little things that make him happy. Obviously the job thing is a major issue, and not one that's easily sorted - and trying to help 'fix' him in that respect too much will just make him feel like he's so far beyond help that it's not worth it - but if he has hobbies or interests, make an effort to get him to follow them. Keeping busy is the best way to avoid wallowing in your own pool of self-pity, and while he's entitled to feel blue about a horrid situation, there's a difference between letting yourself mourn lost oppertunities and drowning in the pool of despair. Maybe he could take a night class, related to his field of work or just something he has always been interested in learning more about but never had the time: right now, it shouldn't seem like he's stuck, but that he has time and space to pursue whatever his dreams may be. If he likes football or something, look into local groups he could join for that (exercise is an amazing way to get the endorphins flowing, gives you goals to meet and makes you feel like you've achieved something, as well as getting him out there meeting people who will like him without feeling obligued to, which helps re-enforce what you're trying to tell him). don't be afraid to push a bit, as he'll likely say "what's the point?", just don't cross over into nagging territory, or it'll come back to the idea of trying to 'fix' him because he's not good enough as he is. A fine line to tread, but it's worth it if you want to help him. And definitely show as much of an interest in it as you can muster, rather than seeing it as a way of pushing him out the door: tag along to a game he's playing, or ask him about his studies, or try and include yourself in some way to reassure him that you're interested in this new direction he's taking.
Really, it boils down to keeping him active and busy and feeling like he's achieveing something more than sitting on his arse hoping the phone will ring (whether or not that's what he's doing). It'll take time, but as long as you're in it for the long haul, you can do it, and he'll feel the effort you're putting in and realise if someone as awesome as you are can be bothered about making him feel better about himself, then there must be something lovable in him.