U S embassy student internships

My son has been selected as an alternate for a summer U S embassy student internship, and he has accepted that place in line. He will have to go through all the vetting required of the first pick, including a security clearance. I understand from here on out it’s mostly a matter of luck, but I’m wondering what percentage of alternates actually end up in the internship. Anybody know? And, assuming he passes the security checks will his security clearance help him in getting jobs with the US government? He will also have non-competitive status because of two years in the Peace Corps, would having that security clearance be helpful beyond the internship?