I would request a meeting with the SENCO at school. The school should be providing support. It isn’t good enough that they forget. Does DS have a SEN support plan - they are called different things in different areas/schools but it is a plan that outlines DC’s needs and the support they require.
You can request an EHCNA yourself. On their website, IPSEA has a model letter you can use. If you do and the LA agree to assess, an OT assessment can be part of the EHCNA.
You can check if your ICB funds sensory OT. If it does, you can look at a referral, it don’t need a diagnosis.
If you want to look privately, you could look at Aniesa Blore, Melinda Eriksen, Diana Pierags, Jo Woods, Donna Podesta, Nikki White, Holly Fenton, Lucy White, Laura Jepson, Children’s Choice OT, Laura Graham, Mary Cunningham, Julia Metcalf, Alex Hermann, Shelly Reid, Sarah Chierico, Dawn Frith, Jenni Cromey. I have tried to include a range of people from across the country. Although many travel for good OTs.
Is DS still under the dietician? If so, I would be pushing them for more support. If not, I would go back to the GP. Do you suspect ARFID? If so, you could look at a referral to a specialist service.