Generally I don't think you should be using the infant hydrating solution that you pictured. It can be too strong for a squirrel, especially if given over several days.
The squirrel should definitely not be given Gatorade. It will make him sicker, not better.
I believe that after you stop feeding the squirrel cereal that the constipation should improve, if not completely go away. This may take a day or two or three.
Fruit shouldn't be the center piece of a captive squirrel's diet. There are a lot of fruits that my squirrel doesn't like either. Don't worry about that. Winter squash, sweet potato, avocado, Greek Yogurt (get the plain, whole milk variety) these are foods that I personally know can make a healthy diet for a grey squirrel. They also contain lots of water.
https://www.henryspets.com/healthy-d...pet-squirrels/
I suggest taking notes about exactly what foods you give at every meal and noting what was eaten. Also note if he pooped or peed. However, the more mobile he gets the harder it will be to track those two. They poop and pee to mark their territory. Also write down the squirrel's energy level and whether the fur is laying flat or standing up. Use your healthy, older squirrel as a point of comparison.
Generally you should try not to change multiple things in the diet at the same time unless the situation is really drastic. When you change multiple things at the same time it is hard to know which things made things better or which things made things worse.
For instance, you squirrel may have pooped because you stopped giving the cereal or maybe because you gave the child's hydrating solution or maybe some other unknown reason.
How hot is it where you keep the squirrel? What is the temperature?