u don't have to nurse whenever he opens his mouth and start to cry. check whether he needs other thing like diaper change, lesser or more clothing, or bulping, or hugging.
you should keep a nursing journal and put down the nursing time, duration, and number of wet and dirty diapers. with this nursing journal, you will know whether you baby is eating enough.
don't nurse every half or one hour, that's too much, even for a nursing mom. if you nurse every 3 hours and your baby sleeps during nursing, wake him up (bulping or diaper change) then continue.