He needs to be hospitalized for proper evaluation, in patients with vomiting and loose stools there can be a few reasons for his low spo2
1. In severe gastroenteritis ppl can develop acute
kidney injury and renal shut down with volume overload
2. The 4 salines could also be the cause of volume overload depending on the ejection fraction of the heart
3. He might actually have a respiratory tract infection as some infections of the respiratory tract present with gastroenteritis too.
All this needs to be evaluated and hence better to hospitalize for further evaluation.