Working during retirement

Here is my question. My husband wants to go ahead and retire he just turned 62. He also wants to work he makes only about $30,000 a year and he has not always made that much. His check would be
around $1200 a year. So how would this work in paying taxes. The lady at SS told us they would hold his first 7 checks what I want to know is then what does he get to keep the next 5. Does he have to pay income tax on those checks we just have not been able to get any clear answers. Thanks if anybody can hlep us.