Is/was he wanting to go? I have Asperger's and I prefer to avoid those things, as do most Aspies I know. Actually in Special Education usually only a few of the non-autistic younger teens ever turn up on school parties.
I'm not saying this is the case with him. And the two of you might have discussed it or he might have indicated wanting to go. Just that a lot of non-autistic people feel sad when they can't go to an event, and a lot of autistic people feel relief.

That said.. can you come together with a group of people and hold a junior prom(/dance/party/dinner/fun evening)?
Or can you and your friend do something together, like a fancy dinner (he in a suite, you in a dress)?

I hope it will be a fun evening anyway.