My Five Year Old Feel Backwards Out Of A Bar

My five year old feel backwards out of a bar stool and hit the the back left side of his head ona chair. It blead a little and has a 3/4" bump. One pupil is slight larger than the other. He cried for about 5 minutes and now is fine. This just happened 30 minutes ago and no vomiting, but the different pupil sizes concerns me. Should I take him to the ER