My 3.5year Old Son Fell Down A Flight Of Steep

My 3.5year old son fell down a flight of steep stairs he skipped on his tailbone most of the way until the bottom he landed on his stomach. He broke his tibia and we now have him in a caste. It happened 2 weeks ago. We though this was all that happened but we are now wondering if there is something going on internally. He has had stomach pain. He has been pooping his pants somewhat regularly which is not normal for him. He has mucousy diarrhea and we noticed small traces of blood just today in his stool. And he has been losing weight and hasn't been eating as much as he used to. It is just dawning on us now that maybe something happened internally due to the fall. We are worried and looking for answers and the next step. Sonogram?