OK, so maybe the outcome of the case was sealed but what about the arrest itself? Can you get a copy of the incident from the arresting officer/precinct via the Freedom of Information Act?
Also - maybe the case is sealed for criminal purposes, but I believe custody hearings run differently. If a judge in a custody hearing asks him if he has ever been arrested, he will have to answer honestly or commit perjury. And if he answers yes, he will have to detail what he got arrested for and how it got dismissed. Get an attorney ASAP and ask.
Besides, if he has already done it once and thinks he got away with it, chances are he thinks he will get away with it again. Keep your eyes open.