Unless you are concerned about his development, there is no need to weigh him at all. If you want to weigh him for your own benefit and comfort, weigh yourself carrying him then weigh youself without him - then deduct one from the other.
I don't think that weighing kids at this age really gives you any benefit, you can see whether he is over or underweight, as long as he has a healthy diet, gets plenty of exercise and is bright, happy and interested in life, then there is nothing to be gained by weighing him.
My 2nd son must have lead bones as he had always weighed consistently more than his build would suggest. When he has gone for his health checks they are always surprised at his weight, but agree that he has a slim build and no excess fat. He has been like this since he was born - you can't always go by the scales.