Re: Scared Sempai
From the brief description, it sounds like part of the problem could simply be a general one of 'zanshin' and all-round awareness of what's happening on the tatami. I have certainly had moments of hesitation on a crowded mat, one one occasion being reassured by an excellent sempai... 'don't worry about whether there's someone behind you; if there is, I won't throw you there'. It could be that he doesn't feel that same level of reassurance...
So perhaps try simplifying the situation - arrange some training sessions with fewer people on the mat, and build that trust in one's training partners which can be slow to develop and quick to erode.