...as i was saying;
in the end i was miserable and my mum or dad, cant remember which, had a conversation with me about it and encouraged me to tell the girl to back off and allow me to speak to who i wished. i wasnt keen as this girl was pretty tough and i was a bit shy and meek but the parent said that if that didnt work they would speak to the teacher for me, which i wasnt sure about either, but couldnt really see any way out so i agreed.
i remember telling her to this day. took me ages to sum up the courage and then i lagged behind after a class till she and i were the only ones there and i said in my strongest voice something like: "you are bullying me and i dont like it. i dont want to be your friend anymore so please leave me alone" (my parent had coached me)
i was really surprised because instead of beating me up, she crumbled. in retrospect i feel really sorry for her, but right then i was just so relieved not to have the threat hanging over me any more.
to complicate things, she then told our teacher that i had bullied her by saying what i said (minus the bullying part) and my teacher, who i trusted and liked, believed her and told me off and asked me to be kind to her and take it back! i guess she never saw the real dynamic between us. anyway...
after that she ceased threatening me and things were a little awkward but i suppose not for too long because we all left for secondary school.
my point is; this kid seems menacing now, but try not to be too hard on him because he is more than likely pretty sad and lonely if this is how he is behaving.
your son needs to show him that he is strong and will not be manipulated. i agree 100% that he should front out the disclosure threat. he should tell the bully "go on then, tell them. i'll deny it and no-one will believe you" even threaten the counter rumours suggested below.
i really think thats about all it will take. they just need to see that your son wont be pushed around and will withdraw his friendship altogether if he doesnt buck up his ideas and start behaving reasonably.
i hope this helps. do post back with his progress