How can you be ordered to pay child support when you were never served with papers??

My fiancé just received a letter from the Nevada office of the district attorney stating he was two months due on child support and he was never even served with papers. He moved back to Tennessee in may of 2012 when he signed the papers. He received a packet of stuff from his ex back in April with copies of her income and expenses and forms to fill out with his income and expenses as well. There were copies of court documents about a hearing on May 6th. He didn't pay too much attention to it because it came from her and he figured if it was important and legit then it would have come from the courts itself. Well so this letter yesterday said he failed to appear in court and his child support was two months behind and he now owes $600.00 on top of the $300.00 a month. How is this possible when he was never served with papers?? Sounds shady to me. And on top of that, she is a nurse and just married a man whose is also a nurse. So their salaries combined are 5 times the amount of his. And she broke rules in the divorce decree and was never penalized even though he contacted her lawyer about it. Any info is greatly appreciated!!