My 16month old son has had black, big, round and thick hard bowel movements. it's almost like he's pooping black rounded rocks. what is this?

Good food, probiotic: If your son overall looks good, appetite and activity are normal and his belly is not tender on pressing, then it is likely just constipation. It is quite common in kids. Give him Probonix probiotics (1 dropperful daily), some sweet potato and avocado. Stop gluten. Reduce milk. His bowels should get better in 2-4 days. If not, then contact his PCP. For severe symptoms, have him be seen asap.