Woah, mothers getting called creepy for calling their sons lovely and enjoying watching them turn from children to young men. A bit creepy in itself, don’t come on a parenting forum if you don’t want to hear such nice things!
I have teen boys and work with teen boys (the joy!) and the posters who say they wouldn’t be happy with their son eating so much, what kind of wierd attitude is that to deprive your own child of food that their body is clearly crying out for. You wouldnt do that to a baby as you’d be arrested, why is it different for a 15 year old? 
those posters criticising or saying such boys will end up obese clearly don’t have any experience of a teenaged boy’s appetite during growth spurts. As a PP said, teenage girls obviously do grow and their appetite does increase (I was so skinny but recall coming home fir school dinners as a teen and my mum giving me a casserole, baked potato and veg. I stil had a big dinner at night too). But boys just seem to experience this growth spurt and demand for energetic to an even greater degree and it is a MARVEL to watch!
I think I get you, OP, it is a nurturing thing you’re describing, that’s joy of knowing you’ve done your job as a mum, your child has had enough food to grow and the growth has accelerated currently to the extent that you can almost see it hapennijg in front of you. It is joyful and fascinating to watch your children become adults. More so if they went through a fussy eater stage as small children. It’s exactly the same lovely warm feeling as when you’re happy at seeing your baby gurgling, smiling and kicking on a changing mat. I just can’t understand those who say it’s creepy. I guess it’s creepy to carry that adoration on when they ARE actually adults, but OP is talking about the between stage when they’re still classed as children and often act as such a lot of the time, but you can see glimpses of the wonderful individual unique human being they will become.
We’re not talking about boys who gorge on junk food and fizzy drinks here either so no problems with obesity for the people so concerned about that. It’s pure appetite and filling a hollow stomach (and legs!)
Lastly, I just want to say that if people don’t believe that teenaged boys don’t grow as fast as is claimed, google stretch marks in adolescent boys. Only discovered this was a thing yesterday because I had a shock when DS (15) showed me back in the bathroom and asked what the marks on his back were. It looked like he’d been mauled by a tiger across the middle of his back!
. I was relieved to discover that yes they are stretch marks but it’s not the same ones as happen when someone puts weight on too quickly. this happens often in tall skinny teenaged boys when they are growing exceptional fast and basically their soft tissues don’t grow as fast as their bones are growing!
It is all fascinating to me experiencing it foe the first time, and realising that none of this teenaged boy appetite stuff is a myth.