I completely understand why this would make you feel a bit sad, and a myriad of other emotions, but it's actually really lovely that other children are celebrating your ds's achievement and want to be a part of that somehow by telling you, so that you can be pleased too.
I work in a school and I see exactly the sort of thing you are taking about. (I also have an autistic child of my own so I see it from that perspective too) Children at the school I work in do the things you are talking about, mollycoddling children that they know have specific difficulties and who they know struggle with certain things.
They are just doing what they are taught to do, and they are expressing that in its purest form, in a way that only children can. They are taught they have to be kind, helpful to people who need help, they need to be pleased for achievement in their class, even if its not their own, they need to accept differences without questioning. And that's what they are doing. They might not want to be his friend, and while it's sad, from their point of view it's probably understandable, especially as they sound like lovely children.
You say that you don't think it's their job to judge how well your ds has done in class, but they won't be able to help that. They will see and notice if something is different to usual, they will hear the praise your ds receives, just the same as they will hear the praise, or sometimes the tellings off, that everyone else receives.