Weight gain works on the basic principle of eating more than what your body can burn - the excess goes into our fat storages. So if your toddler is active and burning up more calories than what he's getting into his tummies, then it's normal that he'll not put on weight.
Also note that some babies pack their growth into height rather than weight. It's normal for most babies to grow length-wise instead of width-wise after their first birthday. They say that babies should reach half of their adult height by the time they are around 5 yo.
So if your kiddo is getting taller (but not busting the scales) and is active plus eating well, I wouldn't worry too much about it (like your paed). If your kiddo is losing weight drastically, then that's something to worry about. Also if he gains too much weight, you have to worry about how to make him lose it! The way I look at it, your kiddo is doing okay.