A calm baby who sleeps at the breast, is regularly filling out, and is displaying no particular sign of unrest, is well fed. Don't go rushing to weigh him before and after each feed!
Weighing him once a week is sufficient to monitor weight gain. Trust your maternal instincts: you will know if your child is doing well or not. A surefire way of knowing: he wets 5 to 6 nappies per day and poos regularly.
Note: The stools of breastfed babies do not resemble the stools of bottlefed babies. They are yellow, hardly smell and may only present themselves as a small stain in the nappy. Don't panic: this is normal. A breastfed baby might not pass stools that frequently (once every three days, for example). It doesn't mean your baby is constipated: the signs of constipation are small hard stools.