I accidentally doubled my 2 y/o dose of hydroxyzine hcl 10mg/5ml. gave him 5ml instead of 2.5. will he be ok?

He'll be fine: A one time dose at that level is not anything to be worried about. You wouldn't want to do it every day, but this is a very safe medicine, and other than he may be a bit drowsy tonight, it isn't going to be a problem. Have a good evening and don't worry about it.

Suspect more sleepy: You can discuss with your pediatrician but may not be a major problem. He may be a little sleepy. He got 5 mg instead of 2.5 mg. For his age the top dose would be 2 mg/kg/day. As you can see it depends on his size. 1 pound equals 2.2 kg