Is he diagnosed? It isn’t entirely clear fron your post.
there is much greater awareness of autism these days, and yes many children/teens are capable of spotting it. If his sister has spotted it then it’s likely others have as well.
in the U.K., it’s generally up to students whether they share knowledge of their diagnoses. It’s quite possible most of his class are assuming he is autistic but doesn’t want to say, while he doesn’t know.
it’s quite likely he is aware that he is “different” in some way. You may find that when you tell him the diagnosis he is relieved as it gives him an explanation for his “difference” and a group that he can access for support and help.
my DD was late diagnosed with adhd and it massively helped her mental health to have what she saw as an explanation.
I understand your worry. However people with autism have autism regardless of whether they tell others or not, and regardless of whether they know or not.
you might be better thinking about how you can support him to develop social skills and move forward from where he is. There is a reasonable amount out there for kids with autism - many voluntary groups such as scouts or guides have groups either specifically for children with additional needs or will support them to be part of a mainstream group and there are activities for children with AN or even specifically autism.