I was thinking more about this last night (sad I know!) probably triggered off by a trip to a park with a river so there were loads of kids/teens who had stripped off. I was initially surprised by how slim the majority were but then of course realised that the overweight ones were possibly not at the park and not stripped off. 
I looked very objectively at my two in amongst their peers and would say that they were not as thin and toned as some of the boys but equally have no trace of the tummy they had and were certainly not overweight. I look back at photos of my youngest at 10/11 and my eldest at 12 and yes, they were overweight then.
I know they weren't eating any differently to what they had been when younger and in fact my younger son was getting podgy on the exact same diet and level of activity that his brother was slim on. They aren't eating any differently or more now than they were then either and I reckon activity levels are more or less the same too.
I have read some of the articles posted above and can absolutely see what it is saying but at the same time, I can't deny my personal experience.
However, it is a fact that your weight is directly related to how much you eat v's how much you move. Energy in/energy out.
My two have always been on the 95-99th percentile for their height - calculations suggest that they will be between 6'3 and 6'6. I have been regular at measuring their height, but less so in measuring their weight as we didn't have decent scales.
When younger they hovered about average (DS1) and about 70th percentile (DS2).
Given the above, I think my two did store fat before puberty and have stretched it off. My sisters boys did the same and they are slim adults, my other sisters son although slighty younger than mine has done the same. What I would say is that my sons have never done the "eat a whole loaf of bread and a gallon of milk" thing when growing, they have eaten much about the same but sometimes that has been stored and then at other times used to fund the growth. Maybe just a different way of growing that some others on this thread have also experienced.
Clearly that isn't going to be the case for all children otherwise we wouldn't have fat teens and adults.
For whatever reason (to avoid being looked down on?) parents of children who didn't stretch out of it haven't posted so it does make it look like it is a probability that this will happen rather than something that might happen iyswim?
Anyway, I hope that the OPs son looks at it as a positive step to getting fitter rather than have it harm his self esteem to the point where he would rather stay in and eat and not exercise to avoid being bullied.