Hmm...I don't know. I guess it's down to the school to communicate with you, about the extent of the issue.
DD2 was actually admitted to hospital with diabetes symptoms and the GP wrote in her referral '?Diabetes, ?Space Occupying Lesion. The Consultant ordered a cannula, CT scan, etc., then suddenly had a change of heart. She realised that DD2 had something quite different.
DD2 had 'Squash Syndrome'. I kid you not. The Consultant said they had seen several children who basically got into a cycle:
a) Drink squash.
b) It tastes nice.
c) Drink more squash.
d) Wee more
e) Get thirsty.....
Soon, you have a child who is fulfilling diabetes 'checklist' criteria:
Drinking lots
Weeing lots
Irritable...etc.
She did suggest that we could limit DD2 to 3 cups of squash per day, then switch to water for the rest of the day. But DD2 found that really quite distressing.
So, we decided that all our children would go cold turkey. DD1 has SN and I thought it would be a real challenge. But I simply said 'no squash now, just water'.
After 3 days, they stopped asking. Now, they are allowed squash at special meals, but they naturally ask for water at all other times.
Only you know if your DS can't drink water. If he would drink water at school if you also made the switch at home, then I think perhaps it would be a good idea to do that. On the other hand, if you think he just won't drink the water under any circumstances, then it will come down to whether it is considered a part of his ASD.
I suspect that there are lots of NT children who would stubbornly refuse water if they got it once they got home, though.