CARROLL COUNTY, Tenn. -- The man charged in the abuse and death of a 7-month-old had his preliminary hearing postponed Wednesday morning. Westlee Yates appeared in Benton County Court on first-degree murder and child abuse charges. "I just want to know what happened to my baby and why," the baby's mother, Tiffany Hilliard, said. "I want to know why he did what he did." Tiffany Hilliard is still trying to comprehend the death of her 7-month-old son, Joseph. "He was a happy baby," Hilliard said. "If he got upset about something, he was very easily entertained." Hilliard's boyfriend is Westlee Yates, and his preliminary hearing is now postponed pending a mental evaluation. "The mental evaluation was not what we expected," said Stacey Myers, the baby's grandmother. "We all knew him, and that's far from who he is." Hilliard, who did not want to talk about their relationship, did say she lived with Yates at a home in Holladay. She says he acted like a stand-up dad. "He has his other two kids," Hilliard said. "He seemed like he was a loving dad." But the family says there were signs. "He had a temper, but we never saw it with the kids," Myers said. "This goes back to what they all say, you never know what happens behind closed doors." Yates' next court appearance has not yet been scheduled pending the mental evaluation. The family says they are planning on setting up a fund in baby Joseph's name to help with the prevention of child abuse.