Popsicles, 100% fruit juice, jello, clear liquids like broth and herbal tea.
No caffeine, no carbonated drinks. They make dehydration worse.
Every time he wakes up offer him a drink. It may hurt to suck the liquid in so if he refuses offer it to him in a spoon. Even a couple of tablespoons of fluids are better than none. I would make sure to call the pediatrician's office and get the doc on call to call me back. The ER cannot give you advice over the phone, they can get sued if something happens to your child, they just won't tell you anything other than things you can look up for yourself.
Your doc should have an on call person 24 hours a day. Call their office and see what the tape says, otherwise you can call the hospital that your docs have privileges at and they'll call the on call doc and have them call you back.
We only have one hospital in the area that we use, I call the switchboard and ask them to have the doc on call from the kids clinic. They call me back usually within a few minutes. They are on call and they know they will get calls. They will give you advice because they are your child's doctors. They may just tell you to go back to the ER though, the ER should be able to run tests and tell if the child is that dehydrated. The ER can also give IV fluids if needed.