Yes dogs can contract Giardia and it is infact something we see often. This can cause vomiting, loss of appetite and almost always diarrhea. This requires a fecal examine for diagnosis.
Intestinal parasites can also cause this which is again a common problem with puppies.
He could just some digestive upset due to something he has eaten outside.
As long as you are sure he has not eaten any non food items and he has not has access to anything toxic then I think it would be OK to try some at home treatment.
1/4 of a 10mg tablet can help to calm the stomach. This can be given every 12 hours. You may also want to try a bland diet for the next 48 hours. A bland diet can be either boiled chicken or boiled hamburger and either rice or sweet potato.
If he continues to have trouble, stops eating or declines in any way then he should be seen by your vet
I hope this helps and he feels better soon. Please reply back if you have additional information and I will be happy to continue.
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