I have 5 brothers and all ate every meal as if it was their last when they were young. Only 1 brother is bigger but he is also a completely different build to the other 4. The eldest who is 60 is as skinny as a rake and has trouble putting on weight.

As you can see everyone has a different sized body so will have different weights. As long as he is eating healthy food, is well and exercises I wouldn't worry.
One thing we did for ourselves was to buy smaller sized plates so we ourselves wouldn't be overeating. We also reduced the size of the meat portion and increased the vegetables.
Having plenty of fruit available for after meal snacks helps. Often when told to have fruit instead of more main course, they suddenly realise they are not really hungry but eating for the sake of it.
In winter we all put on a bit of weight compared to summer as we are more active and outdoors more.