is your scale reliable and was it tared before you weighed him? If you don't tare (reset the scale to zero if you're using a container to weigh him) your weighs wont be accurate.

Also, are you using a digital scale to weigh him or an analog scale? I had an analog scale that i thought was calibrated and it turned out to be more than 100 grams off (in this case it said things were lighter than they actually were).