You should absolutely speak to his HoY or form teacher, explaining that your DS doesn't know you are speaking to him/her. I would hope that they would be able to handle it very sensitively. Has it occurred to you that almost certainly the children calling your son gay are also calling other children gay?
My DH is a secondary teacher and always always picks up pupils who use "gay" as a derogatory term (eg "that's sooo gay"). If he heard a pupil calling another pupil gay he would be down on that pupil like a ton of bricks. His school are strong on zero tolerance of this kind of thing. They have a system where teachers who teach a particular pupil are alerted to be on the look out for this kind of thing, and it's always made very clear if there are any sensitivities, like not making it obvious to the pupil concerned if you are taking action.
My DD started at secondary three weeks ago and we had a settling in meeting last week. Her form tutor asked if she was happy, had made friends, were there any issues etc. Does your school not have a similar thing ?, as this would be the ideal opportunity to discuss this matter.
I'm sorry that your boy is experiencing this because it is the last thing he needs when trying to adapt to a new school.