School has water only policy, son only drinks squash. What are my rights?

Hi,

My son is in mainstream primary school and he has ASD. He is non verbal apart from a few words and has sensory processing disorder. The School he attends has a water only policy for drinks. The trouble is my son only drinks squash. He has a very limited diet.

At first i sent him to school with the fruit squash shots for the ta to put into his water for him to drink. As the weeks have passed they have been giving him less and less squash and now he has water. Well they say he drinks it but i don't believe them as he comes home from school and all he wants is drink drink drink. He also is now constipated all the time which he has never suffered with before and i am sure its connected. He is not toilet trained yet and sees the incontinence team who have agreed without doubt he should have squash. The school still say he can have water as he drinks it but i know this is not the case.

My question is what are my rights with regards to School rules? Can i insist he has squash or do they get overall say? Also how can i know for sure he is getting enough to drink.

If anyone could help i would be must grateful as i seem to be going round in circles with the school and would like to know my rights as a parent.

many thanks, Karen