He does not have to sign, but he must be served with a summons and complaint.
If you can track down his whereabouts, you can get a process server to serve them where he is located.
Once he is served, he must either answer or default.
Please give me a call at 212-370-1660 to arrange for an appointment to discuss the matter at further length.
Leonard M. Weiner, Esq. /Divorce Solutions