Perhaps try talking to him? By letting him know how you feel he might be able to say something to assure you that everything will be okay. Sometimes it's not what's said, but who says it, that can have the best effect on us. It might also depend on the bad thoughts - sometimes it's okay, our mind does tend to wander, but if those thoughts itself make you uncomfortable you might want to talk to someone professionally about it if you aren't sure where to go.