My almost 3 yr old DD2 is still the same weight as she was this time last year, but in that time she has gone from wearing size 1 jeans to size 3 jeans so has really shot up in height.

We're seeing a paed for her about other issues and he has no concerns about her weight. If your DS is happy, energetic and alert then I really think its just one of those kid things where they burn up almost everything they consume just in development, whether that is physical or mental development. Any day now and he'll wake up one morning 5cm taller and 2kg heavier.