Make sure it has water, like you've done, and offer it dog food (the best option for young squirrels) if you can. If he looks really young, you can soften up the dog food with a bit of water. That should hold him off until you can get him to a rescue place.kudos on rescuing the little guy!
If you can't get your hands on dog food, then you can try some nuts (sunflower seeds, pumpkin seeds, peanuts) and maybe a handful of oats and some apple or potato. It'll be better than nothing.