How to manage unruly student who is ruining team

[BulldozerChick]

We are a rookie FTC team and two of the three planned mentors backed out after we started (one had other commitments crop up; one turned out to be a flake). So trying to figure FTC out as one coach and a young rookie team has been stressful (ie losing sleep) enough. However, we recently took on a new student who "has no working memory" is how his mom put it. At the time, I hadn't understood it as a warning. I accept and embrace neurological diversity. But this kid has been SUCH a disruption that it was all I could do not to cry last night. He won't stop talking and only talks louder if one of the kids doesn't immediately respond. I'm ready to call it quits because I don't know what to do. Kids in another room get along great but the kids trying to build near him couldn't get anything done. The stress is too much for me, let alone the other kids. What can I do??? (And I have asked everyone I know involved in FIRST locally if they know of someone who would help mentor, with no luck). I can't do this alone.