<<It's difficult to explain it from the perspective of needs and behaviours because there are so many "he may have trouble waiting his turn, he may get too rough, he may not notice what other people are doing" etc....>>
What about-"My son has some sensory issues...." vague enough, but gets the point across. Its also more accurate, because the term 'autism' is misunderstood by most people, whereas 'sensory issues' is easier to grasp, because it pinpoints the relevant issue in the context of gymnastics.
My understanding of these labels, like autism, aspergers, adhd, sensory processing disorder, is that they are all 'on the spectrum'. they are a collection of symptoms from mild to severe. But the on the ground reality is, that symptoms differ from person to person, and that sensory issues take different forms, and become significant is different contexts.