I think you will need to refer into the Speech Therapists as they can help with children who have sensory issues with food.
SALT will also know who else to refer into in your area
https://sltforkids.co.uk/feeding-clinic/problems-we-help/sensory-based-feeding-difficulties
and also possible an organisation around anxiety with food
https://anxietycare.org.uk/phobias/eating-phobias/
as well as CAMHS
https://www.youngminds.org.uk/about-us/reports-and-impact/policy-reports/what-changes-are-needed-to-improve-young-people-s-eating-disorder-services/
Another post of call will be the autism charities, https://www.autism.org.uk/advice-and-guidance/topics/mental-health/eating-disorders
but this as @niki said above is a long haul and will take some time, covid lockdowns were incredibly difficult with families with ASD kids as it was a struggle to get their food as many won’t eat anything else but what they are familiar with and can get upset if the manufacturer changes the recipe and packaging as it looks different and tastes different
so suggest that you check out ASD, CAMHS and SALT support