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While this is not as toxic as once believed it is probably going to give him stomach upset and he may end up vomiting or getting diarrhea . Drooling is also a symptom because of the sap. Rinse out the mouth with water so the sap is out of the mouth.
here is poinsettia on the ASPCA poison site listing it as non toxic
for upset stomach you can give a pepcid ac or zantac (rantidine) and here is dosing and information http://www.petplace.com/drug-library/ranitidine-hcl-zantac/page1.aspx