How do I politely tell someone to stop prying into my business?

Sorry, but you might want to check yourself and relax about this. I mean really, is it THAT bad? Maybe you could tell him thanks for asking and you appreciate his support and that AS SOON AS YOU HEAR ANYTHING, HE'LL BE THE FIRST TO KNOW. He does not, of course, literally have to be the first one you tell, however, this may slow down his barrage of questions. In any case, it's great that he is trying to help you find a position since it sounds like he might have some good connections if he's in the same field as you. You may also want to consider treating him more like a potential reference because obviously, he could give some very relevant feedback to a potential employer. Additionally, not knowing much about this person, it seems like maybe they don't know much about you and want to chat. Maybe instead of waiting for that inevitable question about YOUR job search, you could ask him about something. Maybe ask him where he went to school or if he has vacation plans. In any case, good luck finding a job and hopefully the next one won't involve any problems for you.