Before answering the question
First of all, you can't help someone who doesn't want it. If he's always rejected your suggestions, doesn't elaborate on what he's looking for or how you can help him, or has never shown being grateful for your help, this might mean he's not interested in your help. If he didn't ask for it in the first place, that's also a good clue. Reflect on those things (or ask him) before putting more effort in his job hunt.
Than there's the fact that he might have unrealistic expectations, or is lazy, about his job hunt. First, he says he doesn't fit all job requirements when he rejects the postings you found, then later tells you he just didn't like the jobs themselves. Someone who lies about why they don't pursue a job, then tell you the reason is because they're picky, is not someone who seems very serious or invested in his quest.
Answering your question
But let's say he wants your help and you still want to give it to him. Maybe it's hard for him to think of specifics, so I'd ask about job posting he has been interested in and wanted to pursue. Maybe also look over a couple of job posting you thought would be great for him, and ask what particularly he didn't like. I'd go back to him and say something like : "I want to help you in your job search, but up until now you've rejected many job openings I forwarded to you. If you'd like me to still help you, I'll need to know more about what you're looking for. Could we maybe look over some job postings you've liked ? I'd like to know more about what kind of jobs and what aspects of a job you're interested in."
If he doesn't seem interested and continues to give you vague responses, I would stop helping him, at least for a while. You can ask questions and offer your help, but in the end it's his job search and up to him to accept your help or not, you can't force him.