Hiya
My DS's, 8 and 3, haven't put on any weight for a year at least, either of them! And I swear I feed them
Doc says not to worry, they are just going upwards and inwards!
When DS1 was 2.5, he didn't then put on any weight til he was 4!!! He was quite a chubby toddler but quite a slim little boy.
The more active they get, mine do swimming, karate, drama club etc, they're not gonna keep the weight on.
As long as he's healthy and thriving, I'm sure he'll be fine.