How much should a 13 year old boy weigh?

For the short answer, look to your grandparents if at all possible. Lots of major genetic traits skip a generation. ON AVERAGE THAT IS, if your parents are of medium height and build, say dad 5'10" - 170lbs and mom 5'5" - 135lbs, but you have a couple of grandparents that are 6'3" or so, and around 200lbs, then it might be perfectly normal for you to be 5'8" and 155lbs at 13yrs, simply cos yer gonna be a big bruiser! Get it?