21mth old will poo pebbles.......why?

Milk can constipate kids ... can you switch the milk to morning and give him water after breakfast? If not, you need to find a veggie that has considerably more fiber in it to help him get softer stools. Another idea would be to give him half the normal amount of milk, and then water, and more milk later, so he is having less milk at a serving. Best of luck to you :)