Try Pedialyte. It's intended for babies during bouts of diarrhea, but kids, even autie kids, seem to like it.

We use Berrilyte or Kinderlyte for hydration issues - they are free of artificial dyes. I think Pedialyte has an unflavored, dye-free option as well.

If your concern is about drinking water, there's a couple things that you can do:
1. Drink only Bottled Water.
2. Get a Filtering system for your home.
3. Buy a PUR or similar Filtering System.

https://i-am-autism.org.uk/navigating-hydration...

Offer water frequently throughout the day. Offer water to individuals with autism frequently throughout the day, even if they do not express thirst. Even smaller sips throughout the day can still help prevent dehydration and ensure that they are getting enough fluids.
Use visual cues. Using visual cues, such as pictures or a water bottle with lines indicating how much water to drink, can really help autistic people to stay in touch with when they need to drink water, and how much.
Experiment with different textures and flavors. If the individual in question seems to dislike the plainness of water, try experimenting with different textures and flavors of water to find one that they like, such as cherry-infused water. Some may prefer carbonated water or flavored water over plain water – whatever works to help get their hydration up in a comfortable way.
Incorporate water into daily routines. People with autism typically dislike change but are big fans of routine, so working towards incorporating drinking water into daily routines can help to keep them on track. Try introducing hydration at specific times of the day, such as after waking up or before and after meals.
Use a straw or water bottle. Autistic people are often quite particular about the sensory inputs they like or loathe. Using a straw or a water bottle they pick out for themselves may make it easier for them to drink more water. Something as simple as the way the water bottle opens can deter an autistic person from using it more frequently, so check in with them to see what they like and take them water bottle shopping!
It is crucial to remember that everyone is different, and what works for one individual with autism may not work for another. It is essential to work with healthcare professionals and caregivers to develop an individualized plan for staying hydrated.