4 months old Decreased milk intake when baby coughing

My baby fell sick since last week ++ and until now still coughing with lots of phlegm. And not sure if its due to that, that now his milk intake dropped. He used to take 150ml/3hrs but now dropped to 100ml only at most. Daily intake around 600ml or so. He’s only 4 months old, on fm. But he’s starting to get a little active back altho still sick. We also went to polyclinic few days back they say its most probably virus that got from IFC. Cause his ifc also always got kids coughing and all. Should i be worried on his decreased intake ? Or its still ok and monitor ? How long usually will they be back to their normal intake ?