First off--Mazel Tov to you and your son!
DS was invited to his 1st Bar Mitzvah last year. I had ABSOLUTELY no idea as to the etiquette, dress code, etc. I called the mom, begged her to have mercy on her Irish Catholic friend, and asked what to do. She was happy to help, and was thrilled that we asked.
In this case, it was a Saturday morning/afternoon affair. DS and his friends wore nice dress pants, button down shirts, ties, and dress shoes. The girls all had on short, dressy dresses.