He freezes up in normal conversation with strangers, can't talk to cashiers/receptionists and even very recently couldn't call out a flight attendant to ask for some water. 

I feel partially responsible for this. I am almost a decade older than him so I've always felt the need to talk to cashiers and strangers for him.. but as he's gotten older, I've tried to make him talk for himself but he says he's scared to do so. 

Even in public places, he just awkwardly fidgets with his hands and looks around to different places. He also slightly shakes his hands when talking which looks really weird and unnatural but I've never said anything about it to him because it might just be his way of reducing anxiety. 

I honestly dont know what to do to help him.