I think it's absolutely discriminatory. They are inviting other children and not him, with the reason being that he has a disability. Will they continue to exclude him as he becomes an adult!? Just because he is child does not make it ok.
I get it, it's your special day, you want it to be perfect ... But to the extent of excluding a family member who may make involuntary noises? Disguisting. My husband and I both have family members with disabilities (one with an intellectual disability who will often say tactless, inappropriate things), and I would never imagine not inviting her to something for fear of her saying inappropriate things or "ruining" the day. How awful for the parents to experience this from their own family.