My 16 month old was constipated and straining for about 24 hours. He had a bm, and there is blood in his stool and on his diaper. Should he see a dr?

Yes. First, his doctor needs to examine him to ensure there is no underlying cause of the constipation and to check for a hemorrhoid or fissure. If you can bring the stool in question with you to the doctor, it's helpful, too -- blood mixed in with the stool can be a sign of intestinal issues. The doctor can also recommend stool softeners for your child.
Anal scratch. When stool hardens or pushes out in a big mass, it may overstretch the anal skin & cause a fissure(crack). The blood is bright red & coats the outside of the stool.It may puddle in a glob or appear like a string of blood. If you can soften the stools with diet change the issue resolves but may take days. Using gentil emoliants like coco butter can provide comfort. Consult with dr if it persists.