How do I know when my child needs medical attention for gastroenteritis?

If he's dehydrated. If your baby is having so much vomiting or diarrhea that he is not urinating well, that may be a symptom of dehydration. Other worrisome symptoms are failure to produce tears when crying, dry inside of the mouth, and lethargy. If your child appears too weak or limp, contact your doctor right away. Also, call if your child is unable to tolerate even tiny amounts at a time of clear liquids.
Fever, dehydration. If your child has a high fever or blood in the stool or if he is vomiting enough or having enough diarrhea that he has a dry mouth, no tears and is very listless, he needs medical attention. If he is less than 2 months old, you need to at least talk to a doctor and might need to have him seen.