Why did the pet ER not keep him? Normally, those places keep them until the morning when they ask you to come get them to take them to your regular vet, but that way they can keep them under observation for the hours immediately following the injury. Nothing against you, but the average owner doesn't know what to look for in terms of the animal going into shock or having problems of any kind, you can just go by the immediate outward signs. I would call my regular vet, as Ginger suggested, and see if there is an emergency number listed, and then all that number, tell them what happened, and ask if you can bring him in to be looked at again. Tell them what the other vet told you about how he feels Mio should be basically up and around by now, and see what they say. If Mio sustained a head injury he could have some serious problems that you might not recognize because you are not trained to look for them.
If you are not able to get anywhere with the emergency number, them I would pack Mio into a carrier and be sitting at your vet's office waiting for him to open the door in the morning. Don't even call first, just meet him there. A possible HBC (hit-by-car) is an emergency with ANY breed, but with a tiny breed like a Chi it's even moreso.
Please post back later today and let us know how Mio is doing.
Ghilly
If you rang your regular vet now, would you get an out-of-hours emergency number for an on-call vet from their surgery, in the middle of the night? (We get that here in UK, don't know if it's the same where you are)
The reason I'm asking is it might be a better idea to do that than to go back to the ER. However, if not, I would take him back for another assessment, or take him to your nearest animal hospital.
He might be fine, just recovering from the trauma. Of course, he may be just tired. It is night time, and he's been through a lot today. I can imagine he won't feel hungry, as concussion can cause nausea. Concussion doesn't go away instantly, it could last a while. The brain has been shunted about in the skull, and needs time to heal. However you need to be sure there is no bleeding going on internally.
With respect, the ER doctor is giving you standard answers I'm sure, over the phone. No-one knows your dog better than you do. Go without sleep tonight, and take your dog back. He might need a CT scan. Don't worry about the ER vet thinking you are fussing. Better to do that than to be sorry if something goes wrong.