I'm impressed by your commitment to improving your work. It's more than most people expect, I think.

One thing that struck me - you said there's no point in asking him if he just thinks you suck. But good coders (he seems to be one) don't learn people-management skills by magic. I wonder if there's any utility to saying something like "I'm trying to figure out what particular areas I can improve the most and which I've got under control. You seem to think my work is acceptable, overall, but the only specific feedback is negative" and then asking about a specific project that you think you did pretty well on. Maybe he thinks you're great but has the geek tendency to pick things apart and find all the flaws.

Or maybe it's best to work on improving the things that interest you and then waiting for annual review season to ask that sort of question. Who knows...