What Causes Hard Dark Speckles In Stools?

My two year and three month old son was diagnosed with autism two weeks ago. For the last 10 days he has been casein free and mostly gluten free. Since changing his diet he has eaten almost nothing but has drunk lots of rice milk. He had not passed a stool since early Sunday then just this evening passed small hard pellety stools. What concerned me is the stools contained small black speckles, evenly distributed among the stools. They don t look like anything he could have eaten in the last 2 days. The speckles are small like coffee grinds and very hard. Do you have any idea what this could be?