Hi,
My mum came out when I was 11 and had just started senior school.
It was a difficult time. I think at that age you are coming to terms with who you are, without having to deal with something else if that makes sense?
I had the usual name calling, " your mums a dyke" yadda yadda, but if I am honest I think every kid got it at some point - black/white, skinny/fat, rich/poor, thick/clever etc etc. At the time I thought it was bad, but really, it was no worse than anyone else got.
I know it's a cliche but the novelty will wear off eventually, it's hard but by not reacting, and just saying "yeah so what" means that it isn't a big deal.
I did find it a lot easier when I got to 13/14 and had a bit more self confidence.
I agree that the school need to step up. It is bullying, and as someone pointed out, if it was racism then it would be stamped out immediately.
The one thing I would urge him to do is be honest. My little sister was 10 at the time, and she would say my mums partner was her "friend/cousin/ whatever" which just made things worse.
Just be there for him, encourage him to talk to you ( sounds like you have good relationship already), and perhaps try and get some of his friends/ their parents on side?
So if this is a bit of a ramble, if I cam be of anymore help just shout.