This is such a tough call, I really feel your pain. I did push DS this year to do a competition, but I didn't last year. I happened to have made the right choice (he was fine once he got up there, and I think he's glad he did it) but I could easily have messed up.
I will say this: I knew he was capable, wasn't going to mess up and was going to do well (which wasn't the case the last year) and I'd worked with him for a significant length of time scripting how he was supposed to act in the competition. His protestations were also sounding a bit thin, almost perfunctory.
To me, this is the most, most difficult part of parenting a kid on the spectrum, because - at least with my son - if I don't do a little pushing or nudging, he'll never try anything. If I push too much, he falls apart. I rarely get terribly good insight into which way I'm going - and, frankly, teachers are no help at all in that regard; they are just as all over the board with their expectations.
Ultimately, the decision is up to you and you have to trust your judgement. You know your child; your teachers only know a small percentage of what you know.