There is no specific weight that a 15-year-old boy should weigh (or a person of any age). This is because weight is related to height and body composition rather than age.
Here are some examples form WikiAnswer users:
The average weight is 126 pounds at an average height of 5' 7" but if you are taller or shorter than this you may weigh heavier or lighter. I'm about 5' 8 and 140 pounds. I'm about 5' 8 or 5' 9 and I weighed in at 155 pounds and I play running back. I am 126 pounds at a height of 5' 7". If you are taller or shorter than this you may weigh heavier or lighter. Yes, I play football defensive tackle and I weigh 212 lbs and I'm 5'10." I am 15 years old, 5 ft 11ish and 135lbs. 5'3" and 130lbs. I'm a soccer player I'm 15 and I'm 5'6" but I also have to work out so I weigh 191 pounds. I'm 15 and weight 135, I work out allot and I'm 5'7 I'm 15 and weigh 215, 5'6". I am very active, and strong boned. I plan to wrestle later this summer. I'm 15 1/2 I'm about 5'11"to 6' and i weigh 143lbs. I am strong but not like steroid hunk I weigh 150 pounds. I amd 14 im 5'8 ish. Im 160 pounds. Mostly Muscle. Im a big build. I am 15 years old, 5 ft 11ish and 135lbs.
Well, my 8 year old son is 85 pounds, solid, and happy. He eats all day and eats a lot but is happy, active, and healthy. My 6 year old son is 54 pounds and the same goes for him. There is literature out there that says the average height/weight of a male/female child is a certain number but genetic factors play a large part in how large or small a child is and current data does not allow for this variance. My husband is 6'3 and around 260 pounds easy, so my boys are probably going to be huge and larger than other kids their same age. Case in point, my 8 year old plays tackle football with 9, 10, and 11 year olds because he weighs too much to play with the kids his own age. No matter what the studies say, just teach your kids healthy eating habits, exercise, good fluid intake and they will be alright.