What happens at a hearing for a petition to enforce child support?

It depends. If this is the first time he is in court on an enforcement action, he will probably just be admonished, ordered to make payments on the back support plus his current obligation, and then it will be set for a review hearing a few months out to make sure he is in compliance. The more times he shows up for an enforcement hearing not having fulfilled his support obligation the better the likelihood that at some point, he will be arrested at the hearing and incarcerated until he pays a "purge" amount, which is an amount set by the court to be applied to his back support obligation.