Have you tried talking to him about his behaviour and telling him what you have observed. This would need to be done in a casual 'I'm curious' manner. This might help him to express verbally what his fears and anxieties are and provide an opportunity for you to reassure him. If this doesn't help, I suggest asking a librarian about suitable books you can read to him about 'feelings'. There are probably some about 'fears'. Once again reading about feelings and talking about the content in the book, might help him to express his own feelings and anxieties so he no longer feels the need to act them out.