blackcat - is it definitely the noise, or the potential noise, or is it the texture he's thinking of - can he not bear certain textures orally? He might be transferring this to horror of others being able to - and enjoying it. Just asking as it seems he is ok with the chewing if it's not gum? Starbursts might have a different texture, so may not freak him so much? If it is a texture thing then OT can definitely help for desensitizing.
Does he hate loud noises? Just trying to work out why only gum - maybe it's more that he's afraid someone may blow a bubble and he gets a fright when they pop?
If it's noise, maybe a listening program of some kind? Tomatis, The Listening Program, etc. This definitely helped us!
Sensory kids can have both hyper and hypo sensitivities, for e.g. DS8 is hypo to visual input, it takes a lot to full up his "visual stimulation" bucket, but auditory and touch? those buckets are the size of thimbles!
