18 month old hardly eats meals any more?

My 18 month old has never been a good eater. He doesnt even eat close to what you've listed. I've expressed concern before but everyone tells me he must be getting enough calories if he's able to have as much energy as he does. lol. Factors I've known to remarkably reduce his intake are when he's teething or sick. I also have noticed he eats TONS better if theres another toddler eating with him.