My autistic son gets extremely upset when people clap and cheer. how can i help him overcome this?

Over-reacting or : Under-reacting to input from any of the 5 senses is part of autism, tho not part of the dsm diagnosis. If your son's intellectual ability is in the normal range, exposure-response avoidance therapy may work. If not, his aba therapist can help. I've had some success with noise-cancellation headphones (e.g., bose), tho i've also had to use a Risperdal m-tab for some patients for specific events.

Gentle desensitizing: First, i would start with getting him some earplugs or headphones to help block sound in those situations. Second, start by gently clapping or cheering at home when he does something good. Then progress to showing him videos of people clapping etc to get him used to it over time. Help him to learn to clap along and cheer so he can learn what a fun positive thing it is!