Hi Elmo, I remember you & the problems your Son was having, especially as at the time my DD was also having awful problems with a boy at her School, so I can really sympathise its a very tough call & yes, unfair your DS misses out because of the bully, but I can see how it would make him feel awkward going with Mum too at 11.
the only possible saving face solution I can think of, is if theres any way you can volunteer yourself to help the birthday boys Mum out with the party??, that way you're there, but not as his chaperone.
Also is there any chance your DS might stand up to the boy away from School??, in which case, letting him go might actually be the turning point for your DS?.......
I found with my DD she really hated the idea of been forced by the ferral boy into breaking the school rules by fighting him back...........I found the turning point for DD quite by accident, we watched Mall Cop, & it had a big effect on her, bad as it sounds, I'm pleased to say, she hurt him back quite badly the very next day..........didn't stop him ruining the whole classes education though, but thankfully with a big push, he finally did get expelled
if theres any chance your DS might stand up to him away from School, I would let him go, but be close by, the fact he actually wants to go suggests perhaps he's not as afraid as you are??
I feel for you, I know from experience how you must be feeling, but it really might be a positive thing to let him go l[hug]]