Over-eating

When do you start to worry about over eating? This is a new one for me, my 5 year old eats next to nothing. But my 18 month old is another story. He's ALWAYS eating. I haven't been worrying too much, since he's as happy with fruit and veg as a biscuit, but I did start to wonder tonight if he over eats. He had dinner (chicken and vegetable stir-fry) dessert, a slice of bread and then finished his brother's leftovers. He'll nurse before bed. Is it ok to let him eat on demand at this age?