As far as being bullied full stop - it totally depends on the atmosphere within the school, the kids, their families etc...you can never tell.
If there is bullying, it could be about anything at all, and you're better off trying to help your DS feel comfortable and confident with himself rather than trying to hide differences, as that confidence is the best thing ever to get bullies to step back. [Not to say you were planning to get him to hide, there, just advice as it's some people's instinct and it almost never works.]
As far as being bullied for being gay goes, in primary school the insult 'gay' is bandied about but with almost no recognition of its tie to homosexuality. There was a kid in my group of friends at primary who was blatantly gay from a young age [we didn't pick up on it till we were older, but with hindsight it was clear from the beginning] but he never got bullied about being effeminate, hanging around with a group of girls or anything like that.
In secondary school you might get a bit of low-level stuff from years 7-9 but assuming the school is decent it should get stamped out pretty quick if it is there, and beyond that these days most kids tend to be pretty accepting. 15 seemed to be the golden age for coming out in my group of peers [I'm 23] and for all those a few years either side of me.
Like I say anyway - if people are determined to bully they will pick on absolutely anything they can. Much better to concentrate on teaching your kids to be happy with and confident in themselves - and also to know how to deal with bullying if it does happen.