Thanks for the extra info about your Chihuahua
boy. This sort of case is always of concern to be since so often I see dogs such as yours in my clinic who have accidentally (or otherwise) ingested toys, bones
and other objects and more often than not, these need to be removed via surgery.
If he has vomition throughout the night, seems painful or you are just overly worried about his state of health - ease your peace of mind by getting him to your local emergency Vet clinic. Because he is so small - pieces of lego and beads could potentially become stuck within his GI tract. The poop eating is definitely a behavioral problem - but shouldn't cause any immediate health concerns.
There are a few things that could potentially happen from here including a partial chronic obstruction, a full complete obstruction or even something like an intersucception where the bowel telescopes in on itself. It is also possible that your boy may just have a bit of gastroenteritis and manage to pass any lego pieces of beads in their entirety. If you are worried and get your boy to the Vet then they may want to give him fluids, and take several radiographs (plain and contrast ones to help highlight a foreign body). If it seems as though there is something stuck in his gut, the next step will be an exploratory laparotomy (opening up his abdomen) to be able to get the foreign material out.
Best of luck and for any further help please feel free to call on me if you have any further questions.
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Dr M D Edwards