well then an anonymous note, if you are genuine scared
but be careful not to start insulting, accusing and blaming him for things and generally telling him how terrible he is, as it will just get him annoyed, amd will not 'work'
if you want an outcome you much approach it in a certain way, even if that mean biting your tongue.
and also be open to the idea that he may be angry at you - ask him if hes ok, and why he seems to be angry at you all, invite him to discuss any issue he may have too.
dont use phrases like 'whats your problem?' etc