Your poor son. I had a problem with my youngest daughter not too long ago with a bully that lives on the next block over. I shared our story here too,, if you care to read it, here it is,,,,,,,
http://www.yorkietalk.com/forums/showthread.php?t=12860 She not only tormented her here but at school too. This bully was a big girl and for what ever reason just did not like Val. I noticed a big change in her. She didn't want to go outside, she would act funny when I dropped her off at school and I can just tell something was not right. When I would ask her if something was wrong, she would say no and that she just didn't feel like playing. I over heard her talking to my older daughter and she was really scared of this bully. She did want to tell me because she thought it would make the bully even more mean. I so know how you are feeling!!! In our case my daughter took care of it herself and although I don't condone fighting,,,,,, she showed this bully that she was not going to take it anymore. The bulling did stop. Along with Val showing her that she was not going to take it anymore and a nice long chat with her parents, the problem was taken care of!
I do wish you luck. These days you can never be too careful and this kid who wrote this letter sounds really sick and full of hate. I hope the school does something about it. I will pray for your son.